perf_counter: Add event overlow handling

Alternative method of mmap() data output handling that provides
better overflow management and a more reliable data stream.

Unlike the previous method, that didn't have any user->kernel
feedback and relied on userspace keeping up, this method relies on
userspace writing its last read position into the control page.

It will ensure new output doesn't overwrite not-yet read events,
new events for which there is no space left are lost and the
overflow counter is incremented, providing exact event loss
numbers.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 130 insertions, 55 deletions

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 109a95723859..7e9108efd305 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1794,6 +1794,12 @@ static int perf_mmap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
         struct perf_mmap_data *data;
         int ret = VM_FAULT_SIGBUS;
 
+        if (vmf->flags & FAULT_FLAG_MKWRITE) {
+                if (vmf->pgoff == 0)
+                        ret = 0;
+                return ret;
+        }
+
         rcu_read_lock();
         data = rcu_dereference(counter->data);
         if (!data)
@@ -1807,9 +1813,16 @@ static int perf_mmap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
                 if ((unsigned)nr > data->nr_pages)
                         goto unlock;
 
+                if (vmf->flags & FAULT_FLAG_WRITE)
+                        goto unlock;
+
                 vmf->page = virt_to_page(data->data_pages[nr]);
         }
+
         get_page(vmf->page);
+        vmf->page->mapping = vma->vm_file->f_mapping;
+        vmf->page->index   = vmf->pgoff;
+
         ret = 0;
 unlock:
         rcu_read_unlock();
@@ -1862,6 +1875,14 @@ fail:
         return -ENOMEM;
 }
 
+static void perf_mmap_free_page(unsigned long addr)
+{
+        struct page *page = virt_to_page(addr);
+
+        page->mapping = NULL;
+        __free_page(page);
+}
+
 static void __perf_mmap_data_free(struct rcu_head *rcu_head)
 {
         struct perf_mmap_data *data;
@@ -1869,9 +1890,10 @@ static void __perf_mmap_data_free(struct rcu_head *rcu_head)
 
         data = container_of(rcu_head, struct perf_mmap_data, rcu_head);
 
-        free_page((unsigned long)data->user_page);
+        perf_mmap_free_page((unsigned long)data->user_page);
         for (i = 0; i < data->nr_pages; i++)
-                free_page((unsigned long)data->data_pages[i]);
+                perf_mmap_free_page((unsigned long)data->data_pages[i]);
+
         kfree(data);
 }
 
@@ -1908,9 +1930,10 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 }
 
 static struct vm_operations_struct perf_mmap_vmops = {
-        .open  = perf_mmap_open,
-        .close = perf_mmap_close,
-        .fault = perf_mmap_fault,
+        .open           = perf_mmap_open,
+        .close          = perf_mmap_close,
+        .fault          = perf_mmap_fault,
+        .page_mkwrite   = perf_mmap_fault,
 };
 
 static int perf_mmap(struct file *file, struct vm_area_struct *vma)
@@ -1924,7 +1947,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
         long user_extra, extra;
         int ret = 0;
 
-        if (!(vma->vm_flags & VM_SHARED) || (vma->vm_flags & VM_WRITE))
+        if (!(vma->vm_flags & VM_SHARED))
                 return -EINVAL;
 
         vma_size = vma->vm_end - vma->vm_start;
@@ -1983,10 +2006,12 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
         atomic_long_add(user_extra, &user->locked_vm);
         vma->vm_mm->locked_vm += extra;
         counter->data->nr_locked = extra;
+        if (vma->vm_flags & VM_WRITE)
+                counter->data->writable = 1;
+
 unlock:
         mutex_unlock(&counter->mmap_mutex);
 
-        vma->vm_flags &= ~VM_MAYWRITE;
         vma->vm_flags |= VM_RESERVED;
         vma->vm_ops = &perf_mmap_vmops;
 
@@ -2163,11 +2188,38 @@ struct perf_output_handle {
         unsigned long           head;
         unsigned long           offset;
         int                     nmi;
-        int                     overflow;
+        int                     sample;
         int                     locked;
         unsigned long           flags;
 };
 
+static bool perf_output_space(struct perf_mmap_data *data,
+                              unsigned int offset, unsigned int head)
+{
+        unsigned long tail;
+        unsigned long mask;
+
+        if (!data->writable)
+                return true;
+
+        mask = (data->nr_pages << PAGE_SHIFT) - 1;
+        /*
+         * Userspace could choose to issue a mb() before updating the tail
+         * pointer. So that all reads will be completed before the write is
+         * issued.
+         */
+        tail = ACCESS_ONCE(data->user_page->data_tail);
+        smp_rmb();
+
+        offset = (offset - tail) & mask;
+        head   = (head   - tail) & mask;
+
+        if ((int)(head - offset) < 0)
+                return false;
+
+        return true;
+}
+
 static void perf_output_wakeup(struct perf_output_handle *handle)
 {
         atomic_set(&handle->data->poll, POLL_IN);
@@ -2258,12 +2310,57 @@ out:
         local_irq_restore(handle->flags);
 }
 
+static void perf_output_copy(struct perf_output_handle *handle,
+                             const void *buf, unsigned int len)
+{
+        unsigned int pages_mask;
+        unsigned int offset;
+        unsigned int size;
+        void **pages;
+
+        offset          = handle->offset;
+        pages_mask      = handle->data->nr_pages - 1;
+        pages           = handle->data->data_pages;
+
+        do {
+                unsigned int page_offset;
+                int nr;
+
+                nr          = (offset >> PAGE_SHIFT) & pages_mask;
+                page_offset = offset & (PAGE_SIZE - 1);
+                size        = min_t(unsigned int, PAGE_SIZE - page_offset, len);
+
+                memcpy(pages[nr] + page_offset, buf, size);
+
+                len         -= size;
+                buf         += size;
+                offset      += size;
+        } while (len);
+
+        handle->offset = offset;
+
+        /*
+         * Check we didn't copy past our reservation window, taking the
+         * possible unsigned int wrap into account.
+         */
+        WARN_ON_ONCE(((long)(handle->head - handle->offset)) < 0);
+}
+
+#define perf_output_put(handle, x) \
+        perf_output_copy((handle), &(x), sizeof(x))
+
 static int perf_output_begin(struct perf_output_handle *handle,
                              struct perf_counter *counter, unsigned int size,
-                             int nmi, int overflow)
+                             int nmi, int sample)
 {
         struct perf_mmap_data *data;
         unsigned int offset, head;
+        int have_lost;
+        struct {
+                struct perf_event_header header;
+                u64                      id;
+                u64                      lost;
+        } lost_event;
 
         /*
          * For inherited counters we send all the output towards the parent.
@@ -2276,19 +2373,25 @@ static int perf_output_begin(struct perf_output_handle *handle,
         if (!data)
                 goto out;
 
-        handle->data     = data;
-        handle->counter  = counter;
-        handle->nmi      = nmi;
-        handle->overflow = overflow;
+        handle->data    = data;
+        handle->counter = counter;
+        handle->nmi     = nmi;
+        handle->sample  = sample;
 
         if (!data->nr_pages)
                 goto fail;
 
+        have_lost = atomic_read(&data->lost);
+        if (have_lost)
+                size += sizeof(lost_event);
+
         perf_output_lock(handle);
 
         do {
                 offset = head = atomic_long_read(&data->head);
                 head += size;
+                if (unlikely(!perf_output_space(data, offset, head)))
+                        goto fail;
         } while (atomic_long_cmpxchg(&data->head, offset, head) != offset);
 
         handle->offset  = offset;
@@ -2297,55 +2400,27 @@ static int perf_output_begin(struct perf_output_handle *handle,
         if ((offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT))
                 atomic_set(&data->wakeup, 1);
 
+        if (have_lost) {
+                lost_event.header.type = PERF_EVENT_LOST;
+                lost_event.header.misc = 0;
+                lost_event.header.size = sizeof(lost_event);
+                lost_event.id          = counter->id;
+                lost_event.lost        = atomic_xchg(&data->lost, 0);
+
+                perf_output_put(handle, lost_event);
+        }
+
         return 0;
 
 fail:
-        perf_output_wakeup(handle);
+        atomic_inc(&data->lost);
+        perf_output_unlock(handle);
 out:
         rcu_read_unlock();
 
         return -ENOSPC;
 }
 
-static void perf_output_copy(struct perf_output_handle *handle,
-                             const void *buf, unsigned int len)
-{
-        unsigned int pages_mask;
-        unsigned int offset;
-        unsigned int size;
-        void **pages;
-
-        offset          = handle->offset;
-        pages_mask      = handle->data->nr_pages - 1;
-        pages           = handle->data->data_pages;
-
-        do {
-                unsigned int page_offset;
-                int nr;
-
-                nr          = (offset >> PAGE_SHIFT) & pages_mask;
-                page_offset = offset & (PAGE_SIZE - 1);
-                size        = min_t(unsigned int, PAGE_SIZE - page_offset, len);
-
-                memcpy(pages[nr] + page_offset, buf, size);
-
-                len         -= size;
-                buf         += size;
-                offset      += size;
-        } while (len);
-
-        handle->offset = offset;
-
-        /*
-         * Check we didn't copy past our reservation window, taking the
-         * possible unsigned int wrap into account.
-         */
-        WARN_ON_ONCE(((long)(handle->head - handle->offset)) < 0);
-}
-
-#define perf_output_put(handle, x) \
-        perf_output_copy((handle), &(x), sizeof(x))
-
 static void perf_output_end(struct perf_output_handle *handle)
 {
         struct perf_counter *counter = handle->counter;
@@ -2353,7 +2428,7 @@ static void perf_output_end(struct perf_output_handle *handle)
 
         int wakeup_events = counter->attr.wakeup_events;
 
-        if (handle->overflow && wakeup_events) {
+        if (handle->sample && wakeup_events) {
                 int events = atomic_inc_return(&data->events);
                 if (events >= wakeup_events) {
                         atomic_sub(wakeup_events, &data->events);
@@ -2958,7 +3033,7 @@ static void perf_log_throttle(struct perf_counter *counter, int enable)
 }
 
 /*
- * Generic counter overflow handling.
+ * Generic counter overflow handling, sampling.
  */
 
 int perf_counter_overflow(struct perf_counter *counter, int nmi,