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[2003:cb:c704:5d00:d8c2:fbf6:a608:957a]) by smtp.gmail.com with ESMTPSA id g5-20020adfd1e5000000b0020a97e7ba9fsm5533899wrd.92.2022.04.19.05.56.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Apr 2022 05:56:17 -0700 (PDT) Message-ID: <59404249-de0c-c569-d04a-9da38ed14b0a@redhat.com> Date: Tue, 19 Apr 2022 14:56:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred To: Roman Gushchin , linux-mm@kvack.org Cc: Andrew Morton , Dave Chinner , linux-kernel@vger.kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Yang Shi References: <20220416004104.4089743-1-roman.gushchin@linux.dev> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220416004104.4089743-1-roman.gushchin@linux.dev> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 4BB2E80014 X-Stat-Signature: gmc84yutu9oqzfzmi18h35sux1zxo9f1 Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=DPbLkdbm; spf=none (imf02.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1650372983-644039 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: On 16.04.22 02:41, Roman Gushchin wrote: > add_nr_deferred() is often called with next_deferred equal to 0. > For instance, it's happening under low memory pressure for any > shrinkers with a low number of cached objects. A corresponding trace > looks like: > <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \ > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > unused scan count 0 new scan count 0 total_scan 0 \ > last shrinker return val 0 > > <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \ > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > unused scan count 0 new scan count 0 total_scan 0 \ > last shrinker return val 0 > > <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \ > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \ > scan count 0 new scan count 0 total_scan 0 \ > last shrinker return val 0 > > This lead to unnecessary checks and atomic operations, which can be > avoided by checking next_deferred for not being zero before calling > add_nr_deferred(). In this case the mm_shrink_slab_end trace point > will get a potentially slightly outdated "new scan count" value, but > it's totally fine. Sufficient improvement to justify added complexity for anybody reading that code? > > Signed-off-by: Roman Gushchin > --- > mm/vmscan.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index d4a7d2bd276d..19d3d4fa1aad 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > * move the unused scan count back into the shrinker in a > * manner that handles concurrent updates. > */ > - new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > + if (next_deferred) > + new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > + else > + new_nr = nr; > > trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan); > return freed; And if we still want to do this optimization, why not put it into add_nr_deferred()? -- Thanks, David / dhildenb