From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (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 DF28C3112C2 for ; Fri, 27 Feb 2026 03:12:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772161950; cv=none; b=QTFncBWh0wtP/D+KupPtD7X6NZ/wLl6Z+bH+en8Q5JxrX+82jlMxOs3+nwqp3gkGH0HL2X7X2aZ0ACDl2YKu0jpfZmbafSMrKAwjgyjV+2wbUYOwSvG9Nag1egK78i4JpXaglAu41X3tf1CIkktexfJqk1o6cnsqI0jIo4+oiWY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772161950; c=relaxed/simple; bh=YTZ41qUcX+MsVbVFDi4U62gzjG9ZurM7ZdoKLYYSJQo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GD72KZZa3IZJ+KEzjF4UuIMJtbigxXTAF+wFBViKWIUwy0G1Uyj5gF4cFbEX5gLw5tRnfNpgCbFenhNMghmDin6q/ynh+VHi1sO6KoRAqOFbbpPtpCupchhmpf0s5/bUXWChocLGj5v5ZVG/37YUOnakXN2yHl6f9EbAct3w2Ng= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=qp0pBMMa; arc=none smtp.client-ip=95.215.58.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="qp0pBMMa" Message-ID: <97e296ed-ef73-44b7-ab68-3d79749caa47@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772161936; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=e9eSofGPc3u44KFMUEmfCKwDammC+wOoAF3xzjznUw8=; b=qp0pBMMafWvK4Sxmn6zcim7tiwH0QeBbCp9/CCu+LJR5xWQBGthnSeE9jXEotNJfYPTl2W uZtCSCB0UO8mUKkhnD6EczVBh0C2svfSj1nm2Tw5rKtFwyaojmGaqDVPRg7xT0n33lQydx kI/BzpUWIfUfu/zYRrRCDDDt0n+taNQ= Date: Fri, 27 Feb 2026 11:11:57 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5 29/32] mm: memcontrol: prepare for reparenting non-hierarchical stats To: Yosry Ahmed , Shakeel Butt Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, muchun.song@linux.dev, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, harry.yoo@oracle.com, yosry.ahmed@linux.dev, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chenridong@huaweicloud.com, mkoutny@suse.com, akpm@linux-foundation.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, lance.yang@linux.dev, bhe@redhat.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Qi Zheng References: X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Hi Yosry, On 2/26/26 11:16 PM, Yosry Ahmed wrote: >>> Did you measure the impact of making state_local atomic on the flush >>> path? It's a slow path but we've seen pain from it being too slow >>> before, because it extends the critical section of the rstat flush >>> lock. >> >> Qi, please measure the impact on flushing and if no impact then no need to do >> anything as I don't want anymore churn in this series. >> >>> >>> Can we keep this non-atomic and use mod_memcg_lruvec_state() here? It >>> will update the stat on the local counter and it will be added to >>> state_local in the flush path when needed. We can even force another >>> flush in reparent_state_local () after reparenting is completed, if we >>> want to avoid leaving a potentially large stat update pending, as it >>> can be missed by mem_cgroup_flush_stats_ratelimited(). >>> >>> Same for reparent_memcg_state_local(), we can probably use mod_memcg_state()? >> >> Yosry, do you mind sending the patch you are thinking about over this series? > > Honestly, I'd rather squash it into this patch if possible. It avoids > churn in the history (switch to atomics and back), and is arguably > simpler than checking for regressions in the flush path. > > What I have in mind is the diff below (build tested only). Qi, would > you be able to test this? It applies directly on this patch in mm-new: Thank you so much for doing this! I'm willing to squash and test. And I found some issues with the diff: > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index d82dbfcc28057..404565e80cbf3 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -234,11 +234,18 @@ static inline void reparent_state_local(struct > mem_cgroup *memcg, struct mem_cgr > if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) > return; > > + /* > + * Reparent stats exposed non-hierarchically. Flush @memcg's > stats first to > + * read its stats accurately , and conservatively flush @parent's stats > + * after reparenting to avoid hiding a potentially large stat update > + * (e.g. from callers of mem_cgroup_flush_stats_ratelimited()). > + */ > __mem_cgroup_flush_stats(memcg, true); > > - /* The following counts are all non-hierarchical and need to > be reparented. */ > reparent_memcg1_state_local(memcg, parent); > reparent_memcg1_lruvec_state_local(memcg, parent); > + > + __mem_cgroup_flush_stats(parent, true); > } > #else > static inline void reparent_state_local(struct mem_cgroup *memcg, > struct mem_cgroup *parent) > @@ -442,7 +449,7 @@ struct lruvec_stats { > long state[NR_MEMCG_NODE_STAT_ITEMS]; > > /* Non-hierarchical (CPU aggregated) state */ > - atomic_long_t state_local[NR_MEMCG_NODE_STAT_ITEMS]; > + long state_local[NR_MEMCG_NODE_STAT_ITEMS]; > > /* Pending child counts during tree propagation */ > long state_pending[NR_MEMCG_NODE_STAT_ITEMS]; > @@ -485,7 +492,7 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec, > return 0; > > pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - x = atomic_long_read(&(pn->lruvec_stats->state_local[i])); > + x = READ_ONCE(pn->lruvec_stats->state_local[i]); > #ifdef CONFIG_SMP > if (x < 0) > x = 0; > @@ -493,6 +500,10 @@ unsigned long lruvec_page_state_local(struct > lruvec *lruvec, > return x; > } > > +static void mod_memcg_lruvec_state(struct lruvec *lruvec, > + enum node_stat_item idx, > + int val); > + > #ifdef CONFIG_MEMCG_V1 > void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg, > struct mem_cgroup *parent, int idx) > @@ -506,12 +517,10 @@ void reparent_memcg_lruvec_state_local(struct > mem_cgroup *memcg, > for_each_node(nid) { > struct lruvec *child_lruvec = mem_cgroup_lruvec(memcg, > NODE_DATA(nid)); > struct lruvec *parent_lruvec = > mem_cgroup_lruvec(parent, NODE_DATA(nid)); > - struct mem_cgroup_per_node *parent_pn; > unsigned long value = > lruvec_page_state_local(child_lruvec, idx); > > - parent_pn = container_of(parent_lruvec, struct > mem_cgroup_per_node, lruvec); > - > - atomic_long_add(value, > &(parent_pn->lruvec_stats->state_local[i])); > + mod_memcg_lruvec_state(child_lruvec, idx, -value); We can't use mod_memcg_lruvec_state() here, because child memcg has already been set CSS_DYING. So in mod_memcg_lruvec_state(), we will get parent memcg. It seems we need to reimplement a function or add a parameter to mod_memcg_lruvec_state() to solve the problem. What do you think? > + mod_memcg_lruvec_state(parent_lruvec, idx, value); > } > } > #endif > @@ -598,7 +607,7 @@ struct memcg_vmstats { > unsigned long events[NR_MEMCG_EVENTS]; > > /* Non-hierarchical (CPU aggregated) page state & events */ > - atomic_long_t state_local[MEMCG_VMSTAT_SIZE]; > + long state_local[MEMCG_VMSTAT_SIZE]; > unsigned long events_local[NR_MEMCG_EVENTS]; > > /* Pending child counts during tree propagation */ > @@ -835,7 +844,7 @@ unsigned long memcg_page_state_local(struct > mem_cgroup *memcg, int idx) > if (WARN_ONCE(BAD_STAT_IDX(i), "%s: missing stat item %d\n", > __func__, idx)) > return 0; > > - x = atomic_long_read(&(memcg->vmstats->state_local[i])); > + x = READ_ONCE(memcg->vmstats->state_local[i]); > #ifdef CONFIG_SMP > if (x < 0) > x = 0; > @@ -852,7 +861,8 @@ void reparent_memcg_state_local(struct mem_cgroup *memcg, > if (WARN_ONCE(BAD_STAT_IDX(i), "%s: missing stat item %d\n", > __func__, idx)) > return; > > - atomic_long_add(value, &(parent->vmstats->state_local[i])); > + mod_memcg_state(memcg, idx, -value); Same as mod_memcg_lruvec_state(). Thanks, Qi > + mod_memcg_state(parent, idx, value); > } > #endif > > @@ -4174,8 +4184,6 @@ struct aggregate_control { > long *aggregate; > /* pointer to the non-hierarchichal (CPU aggregated) counters */ > long *local; > - /* pointer to the atomic non-hierarchichal (CPU aggregated) counters */ > - atomic_long_t *alocal; > /* pointer to the pending child counters during tree propagation */ > long *pending; > /* pointer to the parent's pending counters, could be NULL */ > @@ -4213,12 +4221,8 @@ static void mem_cgroup_stat_aggregate(struct > aggregate_control *ac) > } > > /* Aggregate counts on this level and propagate upwards */ > - if (delta_cpu) { > - if (ac->local) > - ac->local[i] += delta_cpu; > - else if (ac->alocal) > - atomic_long_add(delta_cpu, &(ac->alocal[i])); > - } > + if (delta_cpu) > + ac->local[i] += delta_cpu; > > if (delta) { > ac->aggregate[i] += delta; > @@ -4289,8 +4293,7 @@ static void mem_cgroup_css_rstat_flush(struct > cgroup_subsys_state *css, int cpu) > > ac = (struct aggregate_control) { > .aggregate = memcg->vmstats->state, > - .local = NULL, > - .alocal = memcg->vmstats->state_local, > + .local = memcg->vmstats->state_local, > .pending = memcg->vmstats->state_pending, > .ppending = parent ? parent->vmstats->state_pending : NULL, > .cstat = statc->state, > @@ -4323,8 +4326,7 @@ static void mem_cgroup_css_rstat_flush(struct > cgroup_subsys_state *css, int cpu) > > ac = (struct aggregate_control) { > .aggregate = lstats->state, > - .local = NULL, > - .alocal = lstats->state_local, > + .local = lstats->state_local, > .pending = lstats->state_pending, > .ppending = plstats ? plstats->state_pending : NULL, > .cstat = lstatc->state,