From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1AF5C13C9A9; Fri, 19 Jul 2024 15:07:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721401625; cv=none; b=OpJbwMicCSyHbNHH5xZQ75hwmREnqLwExLiQRuTbVULrclT5eMNgl7A67ccP5JwxSPbVZHiYK/qAJwktlRO2XM/lBamHCT5+POJH4ig2KDBIBtQX67o3xTTZFk/j4eGs0GkwkDAHPwmzG9l7sEttvIivvefcr3G2JmuBa790KlM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721401625; c=relaxed/simple; bh=x04UtHiRnBmbGEix0/mydpwkC0IHcU9T1GhoF957Vmc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=prwJXm1Mq6jZ7gQVjGoXkCRpMC4wYP4P1YO1GoLWFbkQVOKv5+tUOaenNmeWynbrovRGFAcc8WDBqGEyonHdNLKaUgFNG2eQugPU6/DVr03wai5qy5/lCMT3WwY7icFeLAt0qdIlzmh0uWkoYQgkM5KLlwGzNkjirRjKEGu7GWs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AuSyW/Ds; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AuSyW/Ds" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A262CC32782; Fri, 19 Jul 2024 15:07:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1721401624; bh=x04UtHiRnBmbGEix0/mydpwkC0IHcU9T1GhoF957Vmc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=AuSyW/DsXnC3QKZ+0iOmvBiMeLVrz3Pw4vL/gsgBpkKpBiEPTuumYbYNNemIo/jLl sag+tTgidEcB1JX7nfTbvxbyXhoaBuOFt6X/zsfSC4bR7Nw8fd1weqlxbT/CwoJVZ1 Tbcud/Iy+8seN7hOp5NTOSIYQrqdxvVLtIPxwEEIftGEr2KWLajvIdrq55bcmPAJ7I gLdUuZlvf1CzpORzVdRZ9AMItAUBEQQ8SdUJ3S3zTiJUVZ/4MSc/0wpCqw4/YVM/Oo d6Gi02q2OUqP4VA305fuw3Hx7S4iKfxxQDFIUeql+o2LzlCp9MRvCfjp/04RwAnR1d NZ17/7lOgN5aA== Message-ID: <2794de3c-673b-45ea-8897-df1ada9c6717@kernel.org> Date: Fri, 19 Jul 2024 17:07:01 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V7 1/2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes To: Yosry Ahmed Cc: tj@kernel.org, cgroups@vger.kernel.org, shakeel.butt@linux.dev, hannes@cmpxchg.org, lizefan.x@bytedance.com, longman@redhat.com, kernel-team@cloudflare.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <172070450139.2992819.13210624094367257881.stgit@firesoul> Content-Language: en-US From: Jesper Dangaard Brouer In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 17/07/2024 02.30, Yosry Ahmed wrote: > On Thu, Jul 11, 2024 at 6:28 AM 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. >> >> 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 > > 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