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Wed, 23 Aug 2023 11:49:42 +0100 From: David Laight To: 'Jan Kara' , Mateusz Guzik CC: Dennis Zhou , "linux-kernel@vger.kernel.org" , "tj@kernel.org" , "cl@linux.com" , "akpm@linux-foundation.org" , "shakeelb@google.com" , "linux-mm@kvack.org" Subject: RE: [PATCH 0/2] execve scalability issues, part 1 Thread-Topic: [PATCH 0/2] execve scalability issues, part 1 Thread-Index: AQHZ1aeVc0rzaxTOZkOyAPVlJjGUQ6/3shQw Date: Wed, 23 Aug 2023 10:49:41 +0000 Message-ID: References: <20230821202829.2163744-1-mjguzik@gmail.com> <20230821213951.bx3yyqh7omdvpyae@f> <20230822095154.7cr5ofogw552z3jk@quack3> <20230823094915.ggv3spzevgyoov6i@quack3> In-Reply-To: <20230823094915.ggv3spzevgyoov6i@quack3> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 0682340010 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: 3zun3ft9g3kcbd7nhkf983hu19dnur81 X-HE-Tag: 1692787785-948878 X-HE-Meta: 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 wDteEpau NEjwszlIeax0kWCfF5jgE1EFKlAx+yBlve0KxNaqC3U2jYmkbDdEjDOkh7xbRAafP7zsSEzRMWrRgBBMX5gD5HBaLmBogkr8CuLKwefT3djMEXQsdIyEF1Y+R9wh01/dOkatGn51LxcI5jmm3YCBiho5X/MG97kLUIdOnGvZQqY3vtuTVCCb3o3nAM7vQcQNhZ4qToMPVq7/RXj8wgUhoI3+0tX4sX1DFqT64nh00UI5HOd4= 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: 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. > > > > 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. >=20 > arg_lock seems a bit like a hack. How is it related to rss_stat? The sche= me > 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 cac= he > 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.) 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.) =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)