[PATCH] NET : convert IP route cache garbage collection from softirq processing to a workqueue

When the periodic IP route cache flush is done (every 600 seconds on
default configuration), some hosts suffer a lot and eventually trigger
the "soft lockup" message.

dst_run_gc() is doing a scan of a possibly huge list of dst_entries,
eventually freeing some (less than 1%) of them, while holding the
dst_lock spinlock for the whole scan.

Then it rearms a timer to redo the full thing 1/10 s later...
The slowdown can last one minute or so, depending on how active are
the tcp sessions.

This second version of the patch converts the processing from a softirq
based one to a workqueue.

Even if the list of entries in garbage_list is huge, host is still
responsive to softirqs and can make progress.

Instead of resetting gc timer to 0.1 second if one entry was freed in a
gc run, we do this if more than 10% of entries were freed.

Before patch :

Aug 16 06:21:37 SRV1 kernel: BUG: soft lockup detected on CPU#0!
Aug 16 06:21:37 SRV1 kernel:
Aug 16 06:21:37 SRV1 kernel: Call Trace:
Aug 16 06:21:37 SRV1 kernel:  <IRQ>  [<ffffffff802286f0>] wake_up_process+0x10/0x20
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80251e09>] softlockup_tick+0xe9/0x110
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803cd380>] dst_run_gc+0x0/0x140
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff802376f3>] run_local_timers+0x13/0x20
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff802379c7>] update_process_times+0x57/0x90
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80216034>] smp_local_timer_interrupt+0x34/0x60
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff802165cc>] smp_apic_timer_interrupt+0x5c/0x80
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff8020a816>] apic_timer_interrupt+0x66/0x70
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803cd3d3>] dst_run_gc+0x53/0x140
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803cd3c6>] dst_run_gc+0x46/0x140
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80237148>] run_timer_softirq+0x148/0x1c0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff8023340c>] __do_softirq+0x6c/0xe0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff8020ad6c>] call_softirq+0x1c/0x30
Aug 16 06:21:37 SRV1 kernel:  <EOI>  [<ffffffff8020cb34>] do_softirq+0x34/0x90
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff802331cf>] local_bh_enable_ip+0x3f/0x60
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80422913>] _spin_unlock_bh+0x13/0x20
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803dfde8>] rt_garbage_collect+0x1d8/0x320
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803cd4dd>] dst_alloc+0x1d/0xa0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803e1433>] __ip_route_output_key+0x573/0x800
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803c02e2>] sock_common_recvmsg+0x32/0x50
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803e16dc>] ip_route_output_flow+0x1c/0x60
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80400160>] tcp_v4_connect+0x150/0x610
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803ebf07>] inet_bind_bucket_create+0x17/0x60
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff8040cd16>] inet_stream_connect+0xa6/0x2c0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80422981>] _spin_lock_bh+0x11/0x30
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803c0bdf>] lock_sock_nested+0xcf/0xe0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80422981>] _spin_lock_bh+0x11/0x30
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803be551>] sys_connect+0x71/0xa0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803eee3f>] tcp_setsockopt+0x1f/0x30
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803c030f>] sock_common_setsockopt+0xf/0x20
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff803be4bd>] sys_setsockopt+0x9d/0xc0
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff8028881e>] sys_ioctl+0x5e/0x80
Aug 16 06:21:37 SRV1 kernel:  [<ffffffff80209c4e>] system_call+0x7e/0x83

After patch : (RT_CACHE_DEBUG set to 2 to get following traces)

dst_total: 75469 delayed: 74109 work_perf: 141 expires: 150 elapsed: 8092 us
dst_total: 78725 delayed: 73366 work_perf: 743 expires: 400 elapsed: 8542 us
dst_total: 86126 delayed: 71844 work_perf: 1522 expires: 775 elapsed: 8849 us
dst_total: 100173 delayed: 68791 work_perf: 3053 expires: 1256 elapsed: 9748 us
dst_total: 121798 delayed: 64711 work_perf: 4080 expires: 1997 elapsed: 10146 us
dst_total: 154522 delayed: 58316 work_perf: 6395 expires: 25 elapsed: 11402 us
dst_total: 154957 delayed: 58252 work_perf: 64 expires: 150 elapsed: 6148 us
dst_total: 157377 delayed: 57843 work_perf: 409 expires: 400 elapsed: 6350 us
dst_total: 163745 delayed: 56679 work_perf: 1164 expires: 775 elapsed: 7051 us
dst_total: 176577 delayed: 53965 work_perf: 2714 expires: 1389 elapsed: 8120 us
dst_total: 198993 delayed: 49627 work_perf: 4338 expires: 1997 elapsed: 8909 us
dst_total: 226638 delayed: 46865 work_perf: 2762 expires: 2748 elapsed: 7351 us

I successfully reduced the IP route cache of many hosts by a four factor
thanks to this patch. Previously, I had to disable "ip route flush cache"
to avoid crashes.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 122 insertions, 67 deletions

diff --git a/net/core/dst.c b/net/core/dst.c
index 32267a16e01..38c741ac5d0 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -9,6 +9,7 @@
 #include <linux/errno.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
+#include <linux/workqueue.h>
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/netdevice.h>
@@ -19,50 +20,72 @@
 
 #include <net/dst.h>
 
-/* Locking strategy:
- * 1) Garbage collection state of dead destination cache
- *    entries is protected by dst_lock.
- * 2) GC is run only from BH context, and is the only remover
- *    of entries.
- * 3) Entries are added to the garbage list from both BH
- *    and non-BH context, so local BH disabling is needed.
- * 4) All operations modify state, so a spinlock is used.
+/*
+ * Theory of operations:
+ * 1) We use a list, protected by a spinlock, to add
+ *    new entries from both BH and non-BH context.
+ * 2) In order to keep spinlock held for a small delay,
+ *    we use a second list where are stored long lived
+ *    entries, that are handled by the garbage collect thread
+ *    fired by a workqueue.
+ * 3) This list is guarded by a mutex,
+ *    so that the gc_task and dst_dev_event() can be synchronized.
  */
-static struct dst_entry         *dst_garbage_list;
 #if RT_CACHE_DEBUG >= 2
 static atomic_t                  dst_total = ATOMIC_INIT(0);
 #endif
-static DEFINE_SPINLOCK(dst_lock);
 
-static unsigned long dst_gc_timer_expires;
-static unsigned long dst_gc_timer_inc = DST_GC_MAX;
-static void dst_run_gc(unsigned long);
+/*
+ * We want to keep lock & list close together
+ * to dirty as few cache lines as possible in __dst_free().
+ * As this is not a very strong hint, we dont force an alignment on SMP.
+ */
+static struct {
+        spinlock_t              lock;
+        struct dst_entry        *list;
+        unsigned long           timer_inc;
+        unsigned long           timer_expires;
+} dst_garbage = {
+        .lock = __SPIN_LOCK_UNLOCKED(dst_garbage.lock),
+        .timer_inc = DST_GC_MAX,
+};
+static void dst_gc_task(struct work_struct *work);
 static void ___dst_free(struct dst_entry * dst);
 
-static DEFINE_TIMER(dst_gc_timer, dst_run_gc, DST_GC_MIN, 0);
+static DECLARE_DELAYED_WORK(dst_gc_work, dst_gc_task);
 
-static void dst_run_gc(unsigned long dummy)
+static DEFINE_MUTEX(dst_gc_mutex);
+/*
+ * long lived entries are maintained in this list, guarded by dst_gc_mutex
+ */
+static struct dst_entry         *dst_busy_list;
+
+static void dst_gc_task(struct work_struct *work)
 {
         int    delayed = 0;
-        int    work_performed;
-        struct dst_entry * dst, **dstp;
+        int    work_performed = 0;
+        unsigned long expires = ~0L;
+        struct dst_entry *dst, *next, head;
+        struct dst_entry *last = &head;
+#if RT_CACHE_DEBUG >= 2
+        ktime_t time_start = ktime_get();
+        struct timespec elapsed;
+#endif
 
-        if (!spin_trylock(&dst_lock)) {
-                mod_timer(&dst_gc_timer, jiffies + HZ/10);
-                return;
-        }
+        mutex_lock(&dst_gc_mutex);
+        next = dst_busy_list;
 
-        del_timer(&dst_gc_timer);
-        dstp = &dst_garbage_list;
-        work_performed = 0;
-        while ((dst = *dstp) != NULL) {
-                if (atomic_read(&dst->__refcnt)) {
-                        dstp = &dst->next;
+loop:
+        while ((dst = next) != NULL) {
+                next = dst->next;
+                prefetch(&next->next);
+                if (likely(atomic_read(&dst->__refcnt))) {
+                        last->next = dst;
+                        last = dst;
                         delayed++;
                         continue;
                 }
-                *dstp = dst->next;
-                work_performed = 1;
+                work_performed++;
 
                 dst = dst_destroy(dst);
                 if (dst) {
@@ -78,38 +101,56 @@ static void dst_run_gc(unsigned long dummy)
                                 continue;
 
                         ___dst_free(dst);
-                        dst->next = *dstp;
-                        *dstp = dst;
-                        dstp = &dst->next;
+                        dst->next = next;
+                        next = dst;
                 }
         }
-        if (!dst_garbage_list) {
-                dst_gc_timer_inc = DST_GC_MAX;
-                goto out;
+
+        spin_lock_bh(&dst_garbage.lock);
+        next = dst_garbage.list;
+        if (next) {
+                dst_garbage.list = NULL;
+                spin_unlock_bh(&dst_garbage.lock);
+                goto loop;
         }
-        if (!work_performed) {
-                if ((dst_gc_timer_expires += dst_gc_timer_inc) > DST_GC_MAX)
-                        dst_gc_timer_expires = DST_GC_MAX;
-                dst_gc_timer_inc += DST_GC_INC;
-        } else {
-                dst_gc_timer_inc = DST_GC_INC;
-                dst_gc_timer_expires = DST_GC_MIN;
+        last->next = NULL;
+        dst_busy_list = head.next;
+        if (!dst_busy_list)
+                dst_garbage.timer_inc = DST_GC_MAX;
+        else {
+                /*
+                 * if we freed less than 1/10 of delayed entries,
+                 * we can sleep longer.
+                 */
+                if (work_performed <= delayed/10) {
+                        dst_garbage.timer_expires += dst_garbage.timer_inc;
+                        if (dst_garbage.timer_expires > DST_GC_MAX)
+                                dst_garbage.timer_expires = DST_GC_MAX;
+                        dst_garbage.timer_inc += DST_GC_INC;
+                } else {
+                        dst_garbage.timer_inc = DST_GC_INC;
+                        dst_garbage.timer_expires = DST_GC_MIN;
+                }
+                expires = dst_garbage.timer_expires;
+                /*
+                 * if the next desired timer is more than 4 seconds in the future
+                 * then round the timer to whole seconds
+                 */
+                if (expires > 4*HZ)
+                        expires = round_jiffies_relative(expires);
+                schedule_delayed_work(&dst_gc_work, expires);
         }
+
+        spin_unlock_bh(&dst_garbage.lock);
+        mutex_unlock(&dst_gc_mutex);
 #if RT_CACHE_DEBUG >= 2
-        printk("dst_total: %d/%d %ld\n",
-               atomic_read(&dst_total), delayed,  dst_gc_timer_expires);
+        elapsed = ktime_to_timespec(ktime_sub(ktime_get(), time_start));
+        printk(KERN_DEBUG "dst_total: %d delayed: %d work_perf: %d"
+                " expires: %lu elapsed: %lu us\n",
+                atomic_read(&dst_total), delayed, work_performed,
+                expires,
+                elapsed.tv_sec * USEC_PER_SEC + elapsed.tv_nsec / NSEC_PER_USEC);
 #endif
-        /* if the next desired timer is more than 4 seconds in the future
-         * then round the timer to whole seconds
-         */
-        if (dst_gc_timer_expires > 4*HZ)
-                mod_timer(&dst_gc_timer,
-                        round_jiffies(jiffies + dst_gc_timer_expires));
-        else
-                mod_timer(&dst_gc_timer, jiffies + dst_gc_timer_expires);
-
-out:
-        spin_unlock(&dst_lock);
 }
 
 static int dst_discard(struct sk_buff *skb)
@@ -154,16 +195,16 @@ static void ___dst_free(struct dst_entry * dst)
 
 void __dst_free(struct dst_entry * dst)
 {
-        spin_lock_bh(&dst_lock);
+        spin_lock_bh(&dst_garbage.lock);
         ___dst_free(dst);
-        dst->next = dst_garbage_list;
-        dst_garbage_list = dst;
-        if (dst_gc_timer_inc > DST_GC_INC) {
-                dst_gc_timer_inc = DST_GC_INC;
-                dst_gc_timer_expires = DST_GC_MIN;
-                mod_timer(&dst_gc_timer, jiffies + dst_gc_timer_expires);
+        dst->next = dst_garbage.list;
+        dst_garbage.list = dst;
+        if (dst_garbage.timer_inc > DST_GC_INC) {
+                dst_garbage.timer_inc = DST_GC_INC;
+                dst_garbage.timer_expires = DST_GC_MIN;
+                schedule_delayed_work(&dst_gc_work, dst_garbage.timer_expires);
         }
-        spin_unlock_bh(&dst_lock);
+        spin_unlock_bh(&dst_garbage.lock);
 }
 
 struct dst_entry *dst_destroy(struct dst_entry * dst)
@@ -251,7 +292,7 @@ static inline void dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 static int dst_dev_event(struct notifier_block *this, unsigned long event, void *ptr)
 {
         struct net_device *dev = ptr;
-        struct dst_entry *dst;
+        struct dst_entry *dst, *last = NULL;
 
         if (dev->nd_net != &init_net)
                 return NOTIFY_DONE;
@@ -259,11 +300,25 @@ static int dst_dev_event(struct notifier_block *this, unsigned long event, void
         switch (event) {
         case NETDEV_UNREGISTER:
         case NETDEV_DOWN:
-                spin_lock_bh(&dst_lock);
-                for (dst = dst_garbage_list; dst; dst = dst->next) {
+                mutex_lock(&dst_gc_mutex);
+                for (dst = dst_busy_list; dst; dst = dst->next) {
+                        last = dst;
+                        dst_ifdown(dst, dev, event != NETDEV_DOWN);
+                }
+
+                spin_lock_bh(&dst_garbage.lock);
+                dst = dst_garbage.list;
+                dst_garbage.list = NULL;
+                spin_unlock_bh(&dst_garbage.lock);
+
+                if (last)
+                        last->next = dst;
+                else
+                        dst_busy_list = dst;
+                for (; dst; dst = dst->next) {
                         dst_ifdown(dst, dev, event != NETDEV_DOWN);
                 }
-                spin_unlock_bh(&dst_lock);
+                mutex_unlock(&dst_gc_mutex);
                 break;
         }
         return NOTIFY_DONE;