[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <2794de3c-673b-45ea-8897-df1ada9c6717@kernel.org>
Date: Fri, 19 Jul 2024 17:07:01 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: tj@...nel.org, cgroups@...r.kernel.org, shakeel.butt@...ux.dev,
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 V7 1/2] cgroup/rstat: Avoid thundering herd problem by
kswapd across NUMA nodes
On 17/07/2024 02.30, Yosry Ahmed wrote:
> On Thu, Jul 11, 2024 at 6:28 AM Jesper Dangaard Brouer <hawk@...nel.org> 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.
>>
>> Help reviewers follow code: __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>
>
> Thanks for working on this, Jesper! I love the data you collected here!
>
> I think the commit subject and message should be changed to better
> describe the patch. This is a patch that exclusively modifies cgroup
> code, yet the subject is about kswapd. This change affects all users
> of rstat flushing.
>
> I think a better subject would be:
> "cgroup/rstat: avoid flushing if there is an ongoing overlapping
> flush" or similar.
>
Took this for V8.
https://lore.kernel.org/all/172139415725.3084888.13770938453137383953.stgit@firesoul
> The commit message should first describe the cgroup change, and then
> use kswapd as a brief example/illustration of how the problem
> manifests in practice. You should also include a brief summary of the
> numbers you collected from prod.
>
Update desc in V8
>> ---
>> V6: https://lore.kernel.org/all/172052399087.2357901.4955042377343593447.stgit@firesoul/
>> V5: https://lore.kernel.org/all/171956951930.1897969.8709279863947931285.stgit@firesoul/
>> V4: https://lore.kernel.org/all/171952312320.1810550.13209360603489797077.stgit@firesoul/
>> 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 | 95 ++++++++++++++++++++++++++++++++++++++-----
>> 2 files changed, 85 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
[...]
>> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
>> index fb8b49437573..fe2a81a310bb 100644
>> --- a/kernel/cgroup/rstat.c
>> +++ b/kernel/cgroup/rstat.c
[...]
>> static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop)
>> @@ -299,6 +316,53 @@ 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)
>> +{
>> + struct cgroup *ongoing;
>> + bool locked;
>> +
>> + /* Check if ongoing flusher is already taking care of this, if
>
> nit: I think commonly the comment would start on a new line after /*.
>
We use this comment style in networking code.
I've updated it to follow this subsystem.
>> + * we are a descendant skip work, but wait for ongoing flusher
>> + * to complete work.
>> + */
>> +retry:
>> + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
>> + if (ongoing && cgroup_is_descendant(cgrp, ongoing)) {
>> + wait_for_completion_interruptible_timeout(
>> + &ongoing->flush_done, MAX_WAIT);
>> + /* TODO: Add tracepoint here */
>> + return false;
>> + }
>> +
>> + locked = __cgroup_rstat_trylock(cgrp, -1);
>> + if (!locked) {
>> + /* Contended: Handle loosing race for ongoing flusher */
>
> nit: losing
>
Thanks for catching this subtle wording issue.
>> + if (!ongoing && READ_ONCE(cgrp_rstat_ongoing_flusher))
>> + goto retry;
>> +
>> + __cgroup_rstat_lock(cgrp, -1, false);
>> + }
>> + /* Obtained lock, record this cgrp as the ongoing flusher */
>
> Do we want a comment here to explain why there could be an existing
> ongoing flusher (i.e. due to multiple ongoing flushers)? I think it's
> not super obvious.
Extended this in V8.
>
>> + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
>> + if (!ongoing) {
>> + reinit_completion(&cgrp->flush_done);
>> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp);
>> + }
>> + return true; /* locked */
>
> Would it be better to explain the return value of the function in the
> comment above it?
>
Fixed this in V8.
>> +}
>> +
>> +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp)
>> +{
>> + /* Detect if we are the ongoing flusher */
>
> I think this is a bit obvious.
>
True, removed comment.
>> + if (cgrp == READ_ONCE(cgrp_rstat_ongoing_flusher)) {
>> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL);
>> + complete_all(&cgrp->flush_done);
>> + }
>> + __cgroup_rstat_unlock(cgrp, -1);
>> +}
>> +
[...]
Thanks for going through and commenting on the code! :-)
--Jesper
Powered by blists - more mailing lists