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From: Kent Overstreet To: Sebastian Andrzej Siewior Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Ben Segall , Daniel Bristot de Oliveira , Dietmar Eggemann , Ingo Molnar , Juri Lelli , Klara Modin , Mel Gorman , Peter Zijlstra , Steven Rostedt , Suren Baghdasaryan , Thomas Gleixner , Valentin Schneider , Vincent Guittot Subject: Re: [PATCH] sched/task_struct: Move alloc_tag to the end of the struct. Message-ID: <7zretxxixkpfxt6lr7x64n67ql2v2qpb7abbbjklclwlu4u2kx@22o5sdlnpkea> References: <20240621102750.oH9p1FQq@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240621102750.oH9p1FQq@linutronix.de> X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: D3F3CC0021 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 3n5bxsgxrrj833n7bbbzcwt6kj5bns7n X-HE-Tag: 1718979666-201751 X-HE-Meta: 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 K+HegIWL Xb+DhCqEC1Vb2p5ozBL/ZXn6Bd2+Zwtmsy87cW3tpcumeOjanKByfqwPlQBvYcynCwI437S97qS+sYlAjWa86hazA18u1rijrW8BvhRKGgr69nyKdAPGwZqhr3Gui+/Up5Mwc9pc4asMdaFrc2FKn1aF85QhmrM+afQA3 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 Fri, Jun 21, 2024 at 12:27:50PM +0200, Sebastian Andrzej Siewior wrote: > The alloc_tag member has been added to task_struct at the very > beginning. This is a pointer and on 64bit architectures it forces 4 byte > padding after `ptrace' and then forcing another another 4 byte padding > after `on_cpu'. A few members later, `se' requires a cacheline aligned > due to struct sched_avg resulting in 52 hole before `se'. > > This is the case on 64bit-SMP architectures. > The 52 byte hole can be avoided by moving alloc_tag away where it > currently resides. > > Move alloc_tag to the end of task_struct. There is likely a hole before > `thread' due to its alignment requirement and the previous members are > likely to be already pointer-aligned. We sure we want it at the end? we do want it on a hot cacheline