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Date: Fri, 28 Jun 2024 11:39:39 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: tj@...nel.org, cgroups@...r.kernel.org, yosryahmed@...gle.com,
hannes@...xchg.org, lizefan.x@...edance.com, longman@...hat.com,
kernel-team@...udflare.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 2/2] cgroup/rstat: Avoid thundering herd problem by
kswapd across NUMA nodes
On 28/06/2024 01.34, Shakeel Butt wrote:
> On Thu, Jun 27, 2024 at 11:18:56PM GMT, Jesper Dangaard Brouer wrote:
>> Avoid lock contention on the global cgroup rstat lock caused by kswapd
>> starting on all NUMA nodes simultaneously. At Cloudflare, we observed
>> massive issues due to kswapd and the specific mem_cgroup_flush_stats()
>> call inlined in shrink_node, which takes the rstat lock.
>>
>> On our 12 NUMA node machines, each with a kswapd kthread per NUMA node,
>> we noted severe lock contention on the rstat lock. This contention
>> causes 12 CPUs to waste cycles spinning every time kswapd runs.
>> Fleet-wide stats (/proc/N/schedstat) for kthreads revealed that we are
>> burning an average of 20,000 CPU cores fleet-wide on kswapd, primarily
>> due to spinning on the rstat lock.
>>
>> To help reviewer follow code: When the Per-CPU-Pages (PCP) freelist is
>> empty,
>
> Remove the "When the Per-CPU-Pages (PCP) freelist is empty" as there are
> a lot more conditions needed for the waking up kswapds which are not
> needed to be explained here. Just "__alloc_pages_slowpath waking up
> kswapds given the allocation context" or similar text should suffice.
>
Agree.
>> __alloc_pages_slowpath calls wake_all_kswapds(), causing all
>> kswapdN threads to wake up simultaneously. The kswapd thread invokes
>> shrink_node (via balance_pgdat) triggering the cgroup rstat flush
>> operation as part of its work. This results in kernel self-induced rstat
>> lock contention by waking up all kswapd threads simultaneously.
>> Leveraging this detail: balance_pgdat() have NULL value in
>> target_mem_cgroup, this cause mem_cgroup_flush_stats() to do flush with
>> root_mem_cgroup.
>>
>> To avoid this kind of thundering herd problem, kernel previously had a
>> "stats_flush_ongoing" concept, but this was removed as part of commit
>> 7d7ef0a4686a ("mm: memcg: restore subtree stats flushing"). This patch
>> reintroduce and generalized the concept to apply to all users of cgroup
>> rstat, not just memcg.
>>
>> If there is an ongoing rstat flush, and current cgroup is a descendant,
>> then it is unnecessary to do the flush. For callers to still see updated
>> stats, wait for ongoing flusher to complete before returning, but add
>> timeout as stats are already inaccurate given updaters keeps running.
>>
>> Fixes: 7d7ef0a4686a ("mm: memcg: restore subtree stats flushing").
>> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
>> ---
>> V3: https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/
>> V2: https://lore.kernel.org/all/171923011608.1500238.3591002573732683639.stgit@firesoul/
>> V1: https://lore.kernel.org/all/171898037079.1222367.13467317484793748519.stgit@firesoul/
>> RFC: https://lore.kernel.org/all/171895533185.1084853.3033751561302228252.stgit@firesoul/
>>
>> include/linux/cgroup-defs.h | 2 +
>> kernel/cgroup/rstat.c | 64 ++++++++++++++++++++++++++++++++++++-------
>> 2 files changed, 55 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
>> index b36690ca0d3f..a33b37514c29 100644
>> --- a/include/linux/cgroup-defs.h
>> +++ b/include/linux/cgroup-defs.h
>> @@ -548,6 +548,8 @@ struct cgroup {
>> #ifdef CONFIG_BPF_SYSCALL
>> struct bpf_local_storage __rcu *bpf_cgrp_storage;
>> #endif
>> + /* completion queue for cgrp_rstat_ongoing_flusher */
>> + struct completion flush_done;
>>
>> /* All ancestors including self */
>> struct cgroup *ancestors[];
>> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
>> index 2a42be3a9bb3..a98af43bdce7 100644
>> --- a/kernel/cgroup/rstat.c
>> +++ b/kernel/cgroup/rstat.c
>> @@ -2,6 +2,7 @@
>> #include "cgroup-internal.h"
>>
>> #include <linux/sched/cputime.h>
>> +#include <linux/completion.h>
>>
>> #include <linux/bpf.h>
>> #include <linux/btf.h>
>> @@ -11,6 +12,8 @@
>>
>> static DEFINE_SPINLOCK(cgroup_rstat_lock);
>> static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_cpu_lock);
>> +static struct cgroup *cgrp_rstat_ongoing_flusher = NULL;
>> +static DECLARE_COMPLETION(cgrp_rstat_flusher_done);
>
> cgrp_rstat_flusher_done is not needed anymore.
>
True, I already fixed this yesterday, when reading the patch email myself.
>>
>> static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu);
>>
>> @@ -312,6 +315,45 @@ static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop)
>> spin_unlock_irq(&cgroup_rstat_lock);
>> }
>>
>> +#define MAX_WAIT msecs_to_jiffies(100)
>> +/* Trylock helper that also checks for on ongoing flusher */
>> +static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp)
>> +{
>> + bool locked = __cgroup_rstat_trylock(cgrp, -1);
>> + if (!locked) {
>> + struct cgroup *cgrp_ongoing;
>> +
>> + /* Lock is contended, lets check if ongoing flusher is already
>> + * taking care of this, if we are a descendant.
>> + */
>> + cgrp_ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
>> + if (cgrp_ongoing && cgroup_is_descendant(cgrp, cgrp_ongoing)) {
>
> I wonder if READ_ONCE() and cgroup_is_descendant() needs to happen
> within in rcu section. On a preemptable kernel, let's say we got
> preempted in between them, the flusher was unrelated and got freed
> before we get the CPU. In that case we are accessing freed memory.
>
I have to think about this some more.
>> + wait_for_completion_interruptible_timeout(
>> + &cgrp_ongoing->flush_done, MAX_WAIT);
>> +
>> + return false;
>> + }
>> + __cgroup_rstat_lock(cgrp, -1, false);
>> + }
>> + /* Obtained lock, record this cgrp as the ongoing flusher */
>> + if (!READ_ONCE(cgrp_rstat_ongoing_flusher)) {
>
> Can the above condition will ever be false?
>
Yes, I think so, because I realized that cgroup_rstat_flush_locked() can
release/"yield" the lock. Thus, other CPUs/threads have a chance to
call cgroup_rstat_flush, and try to become the "ongoing-flusher".
With this realization, my __cgroup_rstat_trylock() "signal" to detect
ongoing-flushers is also not a good signal. I think we/I should move
the ongoing_flusher detection before attempting to aquire the lock.
If doing so, I'm considering adding a tracepoint after
wait_for_completion() to help us see if code is behaving as expected in
prod env.
>> + reinit_completion(&cgrp->flush_done);
>> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp);
>> + }
>> +
>> + return true; /* locked */
>> +}
>> +
>> +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp)
>> +{
>> + /* Detect if we are the ongoing flusher */
>> + if (cgrp == READ_ONCE(cgrp_rstat_ongoing_flusher)) {
>
> Same.
Same explaination as above.
>
>> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL);
>> + complete_all(&cgrp->flush_done);
>> + }
>> + __cgroup_rstat_unlock(cgrp, -1);
>> +}
>> +
>> /* see cgroup_rstat_flush() */
>> static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
>> __releases(&cgroup_rstat_lock) __acquires(&cgroup_rstat_lock)
>> @@ -361,18 +403,13 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
>> */
>> __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
>> {
>> - bool locked;
>> -
>> might_sleep();
>>
>> - locked = __cgroup_rstat_trylock(cgrp, -1);
>> - if (!locked) {
>> - /* Opportunity to ongoing flush detection */
>> - __cgroup_rstat_lock(cgrp, -1, false);
>> - }
>> + if (!cgroup_rstat_trylock_flusher(cgrp))
>> + return;
>>
>> cgroup_rstat_flush_locked(cgrp);
>> - __cgroup_rstat_unlock(cgrp, -1);
>> + cgroup_rstat_unlock_flusher(cgrp);
>> }
>>
>> /**
>> @@ -388,8 +425,11 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
>> __acquires(&cgroup_rstat_lock)
>> {
>> might_sleep();
>> - __cgroup_rstat_lock(cgrp, -1, true);
>> - cgroup_rstat_flush_locked(cgrp);
>> +
>> + if (cgroup_rstat_trylock_flusher(cgrp))
>> + cgroup_rstat_flush_locked(cgrp);
>> + else
>> + __cgroup_rstat_lock(cgrp, -1, true);
>> }
>>
>> /**
>> @@ -399,7 +439,7 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
>> void cgroup_rstat_flush_release(struct cgroup *cgrp)
>> __releases(&cgroup_rstat_lock)
>> {
>> - __cgroup_rstat_unlock(cgrp, -1);
>> + cgroup_rstat_unlock_flusher(cgrp);
>> }
>>
>> int cgroup_rstat_init(struct cgroup *cgrp)
>> @@ -421,6 +461,8 @@ int cgroup_rstat_init(struct cgroup *cgrp)
>> u64_stats_init(&rstatc->bsync);
>> }
>>
>> + init_completion(&cgrp->flush_done);
>> +
>> return 0;
>> }
>>
>>
>>
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