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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1rDRJx-0025qn-IB; Wed, 13 Dec 2023 15:38:49 +0000 Date: Wed, 13 Dec 2023 15:38:49 +0000 From: Matthew Wilcox To: Yosry Ahmed Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: memcg: remove direct use of __memcg_kmem_uncharge_page Message-ID: References: <20231213130414.353244-1-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 7056840007 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: xuxr4xtz8uig3igaresoqsm43wmhn6t7 X-HE-Tag: 1702481939-347695 X-HE-Meta: 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 CEfyBNcO 8l35NVHDKB+Hts58raT+swCel9fs/5gbmoqs8T3zRipPZ8OdXriEDZQhkWrSNgA37yqns7ovn1vi3//jeqjb2nsd/792zAaEx4xfQeYLYANFCQcqfKzZ5+rSFD/ILeWw4mnB4KinD1sC3mOStj72hkmGpcw6nIPN2AYY9C4ylTPC/AcGgg5JbmqOn8+TVqCB0VGa3RhqFEp4Hk95Q0xAKydrgoz9SwzfrRLGILjSA4JrDQflJTRJaQt270w== 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 Wed, Dec 13, 2023 at 07:08:52AM -0800, Yosry Ahmed wrote: > On Wed, Dec 13, 2023 at 7:01 AM Matthew Wilcox wrote: > > > > On Wed, Dec 13, 2023 at 01:04:14PM +0000, Yosry Ahmed wrote: > > > memcg_kmem_uncharge_page() is an inline wrapper around > > > __memcg_kmem_uncharge_page() that checks memcg_kmem_online() before > > > making the function call. Internally, __memcg_kmem_uncharge_page() has a > > > folio_memcg_kmem() check. > > > > > > The only direct user of __memcg_kmem_uncharge_page(), > > > free_pages_prepare(), checks PageMemcgKmem() before calling it to avoid > > > the function call if possible. Move the folio_memcg_kmem() check from > > > __memcg_kmem_uncharge_page() to memcg_kmem_uncharge_page() as > > > PageMemcgKmem() -- which does the same thing under the hood. Now > > > free_pages_prepare() can also use memcg_kmem_uncharge_page(). > > > > I think you've just pessimised all the other places which call > > memcg_kmem_uncharge_page(). It's a matter of probabilities. In > > free_pages_prepare(), most of the pages being freed are not accounted > > to memcg. Whereas in fork() we are absolutely certain that the pages > > were accounted because we accounted them. > > The check was already there for other callers, but it was inside > __memcg_kmem_uncharge_page(). IIUC, the only change for other callers > is an extra call to compound_head(), and they are not hot paths AFAICT > so it shouldn't be noticeable. How can you seriously claim that fork() is not a hot path? > Am I missing something? Perhaps your point is about how branch > prediction works across function call boundaries? or is this not about > performance at all? > > > > > I think this is a bad change.