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From: Roman Gushchin To: Shakeel Butt Cc: Johannes Weiner , Andrew Morton , Michal Hocko , Muchun Song , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team , stable@vger.kernel.org Subject: Re: [PATCH] memcg: drain obj stock on cpu hotplug teardown Message-ID: References: <20250310230934.2913113-1-shakeel.butt@linux.dev> <20250311153032.GB1211411@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 32CF61C0012 X-Stat-Signature: jxydpbumhb74ahkr43h53ccc8ywm5hwy X-Rspamd-Server: rspam09 X-HE-Tag: 1741710507-677461 X-HE-Meta: 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 oknonKTB jlMJOVD6OhNCrErVT/kM+1HWFuTKlNh0GEX8+BbHEO3S3UY+PT2x5sTtvoQuYUYgHTBgsjwhpNoCKSKJxNSMMOzEW1JiTBvCM+I8gznj48UGEsvq5JXQ4auk+gBoEwi8YbDwYZ25PgSIQdccOzrC2F2U8cDEcleUS5w2BtZcJK37J16StVlmjIPYTB1obzUrd/3ZYBEWsM5MtpkAIyqg91KNHOy3tm169ImsEBSz3eMEMpZ4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Mar 11, 2025 at 08:57:54AM -0700, Shakeel Butt wrote: > On Tue, Mar 11, 2025 at 11:30:32AM -0400, Johannes Weiner wrote: > > On Mon, Mar 10, 2025 at 04:09:34PM -0700, Shakeel Butt wrote: > > > Currently on cpu hotplug teardown, only memcg stock is drained but we > > > need to drain the obj stock as well otherwise we will miss the stats > > > accumulated on the target cpu as well as the nr_bytes cached. The stats > > > include MEMCG_KMEM, NR_SLAB_RECLAIMABLE_B & NR_SLAB_UNRECLAIMABLE_B. In > > > addition we are leaking reference to struct obj_cgroup object. > > > > > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > > > Signed-off-by: Shakeel Butt > > > Cc: > > > > Wow, that's old. Good catch. > > > > > --- > > > mm/memcontrol.c | 9 +++++++++ > > > 1 file changed, 9 insertions(+) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 4de6acb9b8ec..59dcaf6a3519 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -1921,9 +1921,18 @@ void drain_all_stock(struct mem_cgroup *root_memcg) > > > static int memcg_hotplug_cpu_dead(unsigned int cpu) > > > { > > > struct memcg_stock_pcp *stock; > > > + struct obj_cgroup *old; > > > + unsigned long flags; > > > > > > stock = &per_cpu(memcg_stock, cpu); > > > + > > > + /* drain_obj_stock requires stock_lock */ > > > + local_lock_irqsave(&memcg_stock.stock_lock, flags); > > > + old = drain_obj_stock(stock); > > > + local_unlock_irqrestore(&memcg_stock.stock_lock, flags); > > > + > > > drain_stock(stock); > > > + obj_cgroup_put(old); > > > > It might be better to call drain_local_stock() directly instead. That > > would prevent a bug of this type to reoccur in the future. > > The issue is drain_local_stock() works on the local cpu stock while here > we are working on a remote cpu cpu which is dead (memcg_hotplug_cpu_dead > is in PREPARE section of hotplug teardown which runs after the cpu is > dead). > > We can safely call drain_stock() on remote cpu stock here but > drain_obj_stock() is a bit tricky as it can __refill_stock() to local cpu > stock and can call __mod_objcg_mlstate to flush stats. Both of these > requires irq disable for NON-RT kernels and thus I added the local_lock > here. > > Anyways I wanted a simple fix for the backports and in parallel I am > working on cleaning up all the stock functions as I plan to add multi > memcg support. Really curious to see patches/more details here, I have some ideas here as well (nothing materialized yet though). Thanks!