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Wed, 23 Aug 2023 05:01:31 -0700 (PDT) In-Reply-To: References: <20230821202829.2163744-1-mjguzik@gmail.com> <20230821213951.bx3yyqh7omdvpyae@f> <20230822095154.7cr5ofogw552z3jk@quack3> <20230823094915.ggv3spzevgyoov6i@quack3> From: Mateusz Guzik Date: Wed, 23 Aug 2023 14:01:31 +0200 Message-ID: Subject: Re: [PATCH 0/2] execve scalability issues, part 1 To: David Laight Cc: Jan Kara , Dennis Zhou , "linux-kernel@vger.kernel.org" , "tj@kernel.org" , "cl@linux.com" , "akpm@linux-foundation.org" , "shakeelb@google.com" , "linux-mm@kvack.org" Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: CC5E020031 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 8tts5d3uknymigrcqs18q8h1ydc7to59 X-HE-Tag: 1692792092-847792 X-HE-Meta: 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 CBRnAKkS Q21kCgqll5rkQbsNPNNso7vadAcfSakg+OxS1Mg/CX0hX0SjBDTbAM9K9LMqWfqH0Xm7v7bLlLZwSRFYuN1udW2SRAZ00Wx1vN0NpkjhOLnfGMzyq+Vd0wxy6ns1betrnxzi0C7GGKQeF+jtormP5Q9BmbXc1/Witono7ru4raYeGMtKwNSVre3aRVhwu/igjTKVvaIiFVG4nq+E6t9lUMKLeNt4U5WZEAnd61mTG6A8Iiav/Xq2y86ulZaAl+zcSuFZctNNF/0zX8K5Lu4mlEgX6NztbbzUE4dVahyXCe9cQvJdhaaO1UI5FjB+URsNWAHYTXxcwDzy5grEztDnZt6dhoOEAAtG3CAbvqmxMI3wEq+C3NYWOub/mauv78VrrwumeP05MYMiNks1wk8yasvG8ln/DfL/wCD2CukxferSTqm4YIG/qrgu7usGVOBIbUmlP7ak5414dOl526zSL1zdZZURr8v2vKHhD 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 8/23/23, David Laight wrote: > From: Jan Kara >> Sent: Wednesday, August 23, 2023 10:49 AM > .... >> > --- a/include/linux/mm_types.h >> > +++ b/include/linux/mm_types.h >> > @@ -737,7 +737,11 @@ struct mm_struct { >> > >> > unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for >> > /proc/PID/auxv */ >> > >> > - struct percpu_counter rss_stat[NR_MM_COUNTERS]; >> > + union { >> > + struct percpu_counter rss_stat[NR_MM_COUNTERS]; >> > + u64 *rss_stat_single; >> > + }; >> > + bool magic_flag_stuffed_elsewhere; > > I wouldn't use a union to save a pointer - it is asking for trouble. > I may need to abandon this bit anyway -- counter init adds counters to a global list and I can't call easily call it like that. >> > >> > struct linux_binfmt *binfmt; >> > >> > >> > Then for single-threaded case an area is allocated for NR_MM_COUNTERS >> > countes * 2 -- first set updated without any synchro by current >> > thread. Second set only to be modified by others and protected with >> > mm->arg_lock. The lock protects remote access to the union to begin >> > with. >> >> arg_lock seems a bit like a hack. How is it related to rss_stat? The >> scheme >> with two counters is clever but I'm not 100% convinced the complexity is >> really worth it. I'm not sure the overhead of always using an atomic >> counter would really be measurable as atomic counter ops in local CPU >> cache >> tend to be cheap. Did you try to measure the difference? > > A separate lock is worse than atomics. > (Although some 32bit arch may have issues with 64bit atomics.) > But in my proposal the separate lock is used to facilitate *NOT* using atomics by the most common consumer -- the only thread. The lock is only used for the transition to multithreaded state for updated by remote parties (both rare compared to updated by current). > I think you'll be surprised just how slow atomic ops are. > Even when present in the local cache. > (Probably because any other copies have to be invalidated.) > Agreed. They have always been super expensive on x86-64 (and continue to be). I keep running to claims they are not, I don't know where that's coming from. -- Mateusz Guzik