cgroups: add a read-only "procs" file similar to "tasks" that shows only unique tgids

struct cgroup used to have a bunch of fields for keeping track of the
pidlist for the tasks file.  Those are now separated into a new struct
cgroup_pidlist, of which two are had, one for procs and one for tasks.
The way the seq_file operations are set up is changed so that just the
pidlist struct gets passed around as the private data.

Interface example: Suppose a multithreaded process has pid 1000 and other
threads with ids 1001, 1002, 1003:
$ cat tasks
1000
1001
1002
1003
$ cat cgroup.procs
1000
$

Signed-off-by: Ben Blum <bblum@google.com>
Signed-off-by: Paul Menage <menage@google.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 186 insertions, 114 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c833d6f23672..2357733a0a80 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -141,6 +141,17 @@ enum {
         CGRP_WAIT_ON_RMDIR,
 };
 
+struct cgroup_pidlist {
+        /* protects the other fields */
+        struct rw_semaphore mutex;
+        /* array of xids */
+        pid_t *list;
+        /* how many elements the above list has */
+        int length;
+        /* how many files are using the current array */
+        int use_count;
+};
+
 struct cgroup {
         unsigned long flags;            /* "unsigned long" so bitops work */
 
@@ -179,14 +190,9 @@ struct cgroup {
          */
         struct list_head release_list;
 
-        /* pids_mutex protects the fields below */
-        struct rw_semaphore pids_mutex;
-        /* Array of process ids in the cgroup */
-        pid_t *tasks_pids;
-        /* How many files are using the current tasks_pids array */
-        int pids_use_count;
-        /* Length of the current tasks_pids array */
-        int pids_length;
+        /* we will have two separate pidlists, one for pids (the tasks file)
+         * and one for tgids (the procs file). */
+        struct cgroup_pidlist tasks, procs;
 
         /* For RCU-protected deletion */
         struct rcu_head rcu_head;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 22db0a7cf1fa..a9433f50e53d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1121,7 +1121,8 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
         INIT_LIST_HEAD(&cgrp->children);
         INIT_LIST_HEAD(&cgrp->css_sets);
         INIT_LIST_HEAD(&cgrp->release_list);
-        init_rwsem(&cgrp->pids_mutex);
+        init_rwsem(&(cgrp->tasks.mutex));
+        init_rwsem(&(cgrp->procs.mutex));
 }
 
 static void init_cgroup_root(struct cgroupfs_root *root)
@@ -1637,15 +1638,6 @@ static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
         return ret;
 }
 
-/* The various types of files and directories in a cgroup file system */
-enum cgroup_filetype {
-        FILE_ROOT,
-        FILE_DIR,
-        FILE_TASKLIST,
-        FILE_NOTIFY_ON_RELEASE,
-        FILE_RELEASE_AGENT,
-};
-
 /**
  * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
  * @cgrp: the cgroup to be checked for liveness
@@ -2343,7 +2335,7 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
 }
 
 /*
- * Stuff for reading the 'tasks' file.
+ * Stuff for reading the 'tasks'/'procs' files.
  *
  * Reading this file can return large amounts of data if a cgroup has
  * *lots* of attached tasks. So it may need several calls to read(),
@@ -2353,27 +2345,106 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
  */
 
 /*
- * Load into 'pidarray' up to 'npids' of the tasks using cgroup
- * 'cgrp'.  Return actual number of pids loaded.  No need to
- * task_lock(p) when reading out p->cgroup, since we're in an RCU
- * read section, so the css_set can't go away, and is
- * immutable after creation.
+ * pidlist_uniq - given a kmalloc()ed list, strip out all duplicate entries
+ * If the new stripped list is sufficiently smaller and there's enough memory
+ * to allocate a new buffer, will let go of the unneeded memory. Returns the
+ * number of unique elements.
  */
-static int pid_array_load(pid_t *pidarray, int npids, struct cgroup *cgrp)
+/* is the size difference enough that we should re-allocate the array? */
+#define PIDLIST_REALLOC_DIFFERENCE(old, new) ((old) - PAGE_SIZE >= (new))
+static int pidlist_uniq(pid_t **p, int length)
 {
-        int n = 0, pid;
+        int src, dest = 1;
+        pid_t *list = *p;
+        pid_t *newlist;
+
+        /*
+         * we presume the 0th element is unique, so i starts at 1. trivial
+         * edge cases first; no work needs to be done for either
+         */
+        if (length == 0 || length == 1)
+                return length;
+        /* src and dest walk down the list; dest counts unique elements */
+        for (src = 1; src < length; src++) {
+                /* find next unique element */
+                while (list[src] == list[src-1]) {
+                        src++;
+                        if (src == length)
+                                goto after;
+                }
+                /* dest always points to where the next unique element goes */
+                list[dest] = list[src];
+                dest++;
+        }
+after:
+        /*
+         * if the length difference is large enough, we want to allocate a
+         * smaller buffer to save memory. if this fails due to out of memory,
+         * we'll just stay with what we've got.
+         */
+        if (PIDLIST_REALLOC_DIFFERENCE(length, dest)) {
+                newlist = krealloc(list, dest * sizeof(pid_t), GFP_KERNEL);
+                if (newlist)
+                        *p = newlist;
+        }
+        return dest;
+}
+
+static int cmppid(const void *a, const void *b)
+{
+        return *(pid_t *)a - *(pid_t *)b;
+}
+
+/*
+ * Load a cgroup's pidarray with either procs' tgids or tasks' pids
+ */
+static int pidlist_array_load(struct cgroup *cgrp, bool procs)
+{
+        pid_t *array;
+        int length;
+        int pid, n = 0; /* used for populating the array */
         struct cgroup_iter it;
         struct task_struct *tsk;
+        struct cgroup_pidlist *l;
+
+        /*
+         * If cgroup gets more users after we read count, we won't have
+         * enough space - tough.  This race is indistinguishable to the
+         * caller from the case that the additional cgroup users didn't
+         * show up until sometime later on.
+         */
+        length = cgroup_task_count(cgrp);
+        array = kmalloc(length * sizeof(pid_t), GFP_KERNEL);
+        if (!array)
+                return -ENOMEM;
+        /* now, populate the array */
         cgroup_iter_start(cgrp, &it);
         while ((tsk = cgroup_iter_next(cgrp, &it))) {
-                if (unlikely(n == npids))
+                if (unlikely(n == length))
                         break;
-                pid = task_pid_vnr(tsk);
-                if (pid > 0)
-                        pidarray[n++] = pid;
+                /* get tgid or pid for procs or tasks file respectively */
+                pid = (procs ? task_tgid_vnr(tsk) : task_pid_vnr(tsk));
+                if (pid > 0) /* make sure to only use valid results */
+                        array[n++] = pid;
         }
         cgroup_iter_end(cgrp, &it);
-        return n;
+        length = n;
+        /* now sort & (if procs) strip out duplicates */
+        sort(array, length, sizeof(pid_t), cmppid, NULL);
+        if (procs) {
+                length = pidlist_uniq(&array, length);
+                l = &(cgrp->procs);
+        } else {
+                l = &(cgrp->tasks);
+        }
+        /* store array in cgroup, freeing old if necessary */
+        down_write(&l->mutex);
+        kfree(l->list);
+        l->list = array;
+        l->length = length;
+        l->use_count++;
+        up_write(&l->mutex);
+        return 0;
 }
 
 /**
@@ -2430,19 +2501,14 @@ err:
         return ret;
 }
 
-static int cmppid(const void *a, const void *b)
-{
-        return *(pid_t *)a - *(pid_t *)b;
-}
-
 
 /*
- * seq_file methods for the "tasks" file. The seq_file position is the
+ * seq_file methods for the tasks/procs files. The seq_file position is the
  * next pid to display; the seq_file iterator is a pointer to the pid
- * in the cgroup->tasks_pids array.
+ * in the cgroup->l->list array.
  */
 
-static void *cgroup_tasks_start(struct seq_file *s, loff_t *pos)
+static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
 {
         /*
          * Initially we receive a position value that corresponds to
@@ -2450,46 +2516,45 @@ static void *cgroup_tasks_start(struct seq_file *s, loff_t *pos)
          * after a seek to the start). Use a binary-search to find the
          * next pid to display, if any
          */
-        struct cgroup *cgrp = s->private;
+        struct cgroup_pidlist *l = s->private;
         int index = 0, pid = *pos;
         int *iter;
 
-        down_read(&cgrp->pids_mutex);
+        down_read(&l->mutex);
         if (pid) {
-                int end = cgrp->pids_length;
+                int end = l->length;
 
                 while (index < end) {
                         int mid = (index + end) / 2;
-                        if (cgrp->tasks_pids[mid] == pid) {
+                        if (l->list[mid] == pid) {
                                 index = mid;
                                 break;
-                        } else if (cgrp->tasks_pids[mid] <= pid)
+                        } else if (l->list[mid] <= pid)
                                 index = mid + 1;
                         else
                                 end = mid;
                 }
         }
         /* If we're off the end of the array, we're done */
-        if (index >= cgrp->pids_length)
+        if (index >= l->length)
                 return NULL;
         /* Update the abstract position to be the actual pid that we found */
-        iter = cgrp->tasks_pids + index;
+        iter = l->list + index;
         *pos = *iter;
         return iter;
 }
 
-static void cgroup_tasks_stop(struct seq_file *s, void *v)
+static void cgroup_pidlist_stop(struct seq_file *s, void *v)
 {
-        struct cgroup *cgrp = s->private;
-        up_read(&cgrp->pids_mutex);
+        struct cgroup_pidlist *l = s->private;
+        up_read(&l->mutex);
 }
 
-static void *cgroup_tasks_next(struct seq_file *s, void *v, loff_t *pos)
+static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
 {
-        struct cgroup *cgrp = s->private;
-        int *p = v;
-        int *end = cgrp->tasks_pids + cgrp->pids_length;
-
+        struct cgroup_pidlist *l = s->private;
+        pid_t *p = v;
+        pid_t *end = l->list + l->length;
         /*
          * Advance to the next pid in the array. If this goes off the
          * end, we're done
@@ -2503,98 +2568,94 @@ static void *cgroup_tasks_next(struct seq_file *s, void *v, loff_t *pos)
         }
 }
 
-static int cgroup_tasks_show(struct seq_file *s, void *v)
+static int cgroup_pidlist_show(struct seq_file *s, void *v)
 {
         return seq_printf(s, "%d\n", *(int *)v);
 }
 
-static const struct seq_operations cgroup_tasks_seq_operations = {
-        .start = cgroup_tasks_start,
-        .stop = cgroup_tasks_stop,
-        .next = cgroup_tasks_next,
-        .show = cgroup_tasks_show,
+/*
+ * seq_operations functions for iterating on pidlists through seq_file -
+ * independent of whether it's tasks or procs
+ */
+static const struct seq_operations cgroup_pidlist_seq_operations = {
+        .start = cgroup_pidlist_start,
+        .stop = cgroup_pidlist_stop,
+        .next = cgroup_pidlist_next,
+        .show = cgroup_pidlist_show,
 };
 
-static void release_cgroup_pid_array(struct cgroup *cgrp)
+static void cgroup_release_pid_array(struct cgroup_pidlist *l)
 {
-        down_write(&cgrp->pids_mutex);
-        BUG_ON(!cgrp->pids_use_count);
-        if (!--cgrp->pids_use_count) {
-                kfree(cgrp->tasks_pids);
-                cgrp->tasks_pids = NULL;
-                cgrp->pids_length = 0;
+        down_write(&l->mutex);
+        BUG_ON(!l->use_count);
+        if (!--l->use_count) {
+                kfree(l->list);
+                l->list = NULL;
+                l->length = 0;
         }
-        up_write(&cgrp->pids_mutex);
+        up_write(&l->mutex);
 }
 
-static int cgroup_tasks_release(struct inode *inode, struct file *file)
+static int cgroup_pidlist_release(struct inode *inode, struct file *file)
 {
-        struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
-
+        struct cgroup_pidlist *l;
         if (!(file->f_mode & FMODE_READ))
                 return 0;
-
-        release_cgroup_pid_array(cgrp);
+        /*
+         * the seq_file will only be initialized if the file was opened for
+         * reading; hence we check if it's not null only in that case.
+         */
+        l = ((struct seq_file *)file->private_data)->private;
+        cgroup_release_pid_array(l);
         return seq_release(inode, file);
 }
 
-static struct file_operations cgroup_tasks_operations = {
+static const struct file_operations cgroup_pidlist_operations = {
         .read = seq_read,
         .llseek = seq_lseek,
         .write = cgroup_file_write,
-        .release = cgroup_tasks_release,
+        .release = cgroup_pidlist_release,
 };
 
 /*
- * Handle an open on 'tasks' file.  Prepare an array containing the
- * process id's of tasks currently attached to the cgroup being opened.
+ * The following functions handle opens on a file that displays a pidlist
+ * (tasks or procs). Prepare an array of the process/thread IDs of whoever's
+ * in the cgroup.
  */
-
-static int cgroup_tasks_open(struct inode *unused, struct file *file)
+/* helper function for the two below it */
+static int cgroup_pidlist_open(struct file *file, bool procs)
 {
         struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
-        pid_t *pidarray;
-        int npids;
+        struct cgroup_pidlist *l = (procs ? &cgrp->procs : &cgrp->tasks);
         int retval;
 
         /* Nothing to do for write-only files */
         if (!(file->f_mode & FMODE_READ))
                 return 0;
 
-        /*
-         * If cgroup gets more users after we read count, we won't have
-         * enough space - tough.  This race is indistinguishable to the
-         * caller from the case that the additional cgroup users didn't
-         * show up until sometime later on.
-         */
-        npids = cgroup_task_count(cgrp);
-        pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
-        if (!pidarray)
-                return -ENOMEM;
-        npids = pid_array_load(pidarray, npids, cgrp);
-        sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
-
-        /*
-         * Store the array in the cgroup, freeing the old
-         * array if necessary
-         */
-        down_write(&cgrp->pids_mutex);
-        kfree(cgrp->tasks_pids);
-        cgrp->tasks_pids = pidarray;
-        cgrp->pids_length = npids;
-        cgrp->pids_use_count++;
-        up_write(&cgrp->pids_mutex);
-
-        file->f_op = &cgroup_tasks_operations;
+        /* have the array populated */
+        retval = pidlist_array_load(cgrp, procs);
+        if (retval)
+                return retval;
+        /* configure file information */
+        file->f_op = &cgroup_pidlist_operations;
 
-        retval = seq_open(file, &cgroup_tasks_seq_operations);
+        retval = seq_open(file, &cgroup_pidlist_seq_operations);
         if (retval) {
-                release_cgroup_pid_array(cgrp);
+                cgroup_release_pid_array(l);
                 return retval;
         }
-        ((struct seq_file *)file->private_data)->private = cgrp;
+        ((struct seq_file *)file->private_data)->private = l;
         return 0;
 }
+static int cgroup_tasks_open(struct inode *unused, struct file *file)
+{
+        return cgroup_pidlist_open(file, false);
+}
+static int cgroup_procs_open(struct inode *unused, struct file *file)
+{
+        return cgroup_pidlist_open(file, true);
+}
 
 static u64 cgroup_read_notify_on_release(struct cgroup *cgrp,
                                             struct cftype *cft)
@@ -2617,21 +2678,27 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
 /*
  * for the common functions, 'private' gives the type of file
  */
+/* for hysterical raisins, we can't put this on the older files */
+#define CGROUP_FILE_GENERIC_PREFIX "cgroup."
 static struct cftype files[] = {
         {
                 .name = "tasks",
                 .open = cgroup_tasks_open,
                 .write_u64 = cgroup_tasks_write,
-                .release = cgroup_tasks_release,
-                .private = FILE_TASKLIST,
+                .release = cgroup_pidlist_release,
                 .mode = S_IRUGO | S_IWUSR,
         },
-
+        {
+                .name = CGROUP_FILE_GENERIC_PREFIX "procs",
+                .open = cgroup_procs_open,
+                /* .write_u64 = cgroup_procs_write, TODO */
+                .release = cgroup_pidlist_release,
+                .mode = S_IRUGO,
+        },
         {
                 .name = "notify_on_release",
                 .read_u64 = cgroup_read_notify_on_release,
                 .write_u64 = cgroup_write_notify_on_release,
-                .private = FILE_NOTIFY_ON_RELEASE,
         },
 };
 
@@ -2640,7 +2707,6 @@ static struct cftype cft_release_agent = {
         .read_seq_string = cgroup_release_agent_show,
         .write_string = cgroup_release_agent_write,
         .max_write_len = PATH_MAX,
-        .private = FILE_RELEASE_AGENT,
 };
 
 static int cgroup_populate_dir(struct cgroup *cgrp)