From: Will Deacon <will@kernel.org>
To: Hou Tao <houtao1@huawei.com>
Cc: peterz@infradead.org, Oleg Nesterov <oleg@redhat.com>,
Ingo Molnar <mingo@redhat.com>, Dennis Zhou <dennis@kernel.org>,
Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Jan Kara <jack@suse.cz>
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count
Date: Thu, 17 Sep 2020 09:48:31 +0100 [thread overview]
Message-ID: <20200917084831.GA29295@willie-the-truck> (raw)
In-Reply-To: <b885ce8e-4b0b-8321-c2cc-ee8f42de52d4@huawei.com>
On Wed, Sep 16, 2020 at 08:32:20PM +0800, Hou Tao wrote:
> > Subject: locking/percpu-rwsem: Use this_cpu_{inc,dec}() for read_count
> > From: Hou Tao <houtao1@huawei.com>
> > Date: Tue, 15 Sep 2020 22:07:50 +0800
> >
> > From: Hou Tao <houtao1@huawei.com>
> >
> > The __this_cpu*() accessors are (in general) IRQ-unsafe which, given
> > that percpu-rwsem is a blocking primitive, should be just fine.
> >
> > However, file_end_write() is used from IRQ context and will cause
> > load-store issues.
> >
> > Fixing it by using the IRQ-safe this_cpu_*() for operations on
> > read_count. This will generate more expensive code on a number of
> > platforms, which might cause a performance regression for some of the
> > other percpu-rwsem users.
> >
> > If any such is reported, we can consider alternative solutions.
> >
> I have simply test the performance impact on both x86 and aarch64.
>
> There is no degradation under x86 (2 sockets, 18 core per sockets, 2 threads per core)
>
> v5.8.9
> no writer, reader cn | 18 | 36 | 72
> the rate of down_read/up_read per second | 231423957 | 230737381 | 109943028
> the rate of down_read/up_read per second (patched) | 232864799 | 233555210 | 109768011
>
> However the performance degradation is huge under aarch64 (4 sockets, 24 core per sockets): nearly 60% lost.
>
> v4.19.111
> no writer, reader cn | 24 | 48 | 72 | 96
> the rate of down_read/up_read per second | 166129572 | 166064100 | 165963448 | 165203565
> the rate of down_read/up_read per second (patched) | 63863506 | 63842132 | 63757267 | 63514920
>
> I will test the aarch64 host by using v5.8 tomorrow.
Thanks. We did improve the preempt_count() munging a bit since 4.19 (I
think), so maybe 5.8 will be a bit better. Please report back!
Will
next prev parent reply other threads:[~2020-09-17 8:48 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-15 14:07 [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count Hou Tao
2020-09-15 15:06 ` peterz
2020-09-15 15:31 ` Oleg Nesterov
2020-09-15 15:51 ` peterz
2020-09-15 16:03 ` peterz
2020-09-15 16:11 ` Will Deacon
2020-09-15 18:11 ` peterz
2020-09-16 8:20 ` Will Deacon
2020-09-15 16:47 ` Oleg Nesterov
2020-09-16 12:32 ` Hou Tao
2020-09-16 12:51 ` peterz
2020-09-17 8:48 ` Will Deacon [this message]
2020-09-24 11:55 ` Hou Tao
2020-09-29 17:49 ` Will Deacon
2020-09-29 18:07 ` Ard Biesheuvel
2020-09-17 10:51 ` Boaz Harrosh
2020-09-17 12:01 ` Oleg Nesterov
2020-09-17 12:48 ` Matthew Wilcox
2020-09-17 13:22 ` peterz
2020-09-17 13:34 ` Oleg Nesterov
2020-09-17 13:46 ` Boaz Harrosh
2020-09-17 14:46 ` Christoph Hellwig
2020-09-18 9:07 ` Jan Kara
2020-09-18 10:01 ` peterz
2020-09-18 10:04 ` peterz
2020-09-18 10:07 ` peterz
2020-09-18 10:12 ` peterz
2020-09-18 10:48 ` Oleg Nesterov
2020-09-18 11:03 ` peterz
2020-09-18 13:09 ` Oleg Nesterov
2020-09-18 13:26 ` Jan Kara
2020-09-20 23:49 ` Dave Chinner
2020-09-18 8:36 ` [tip: locking/urgent] locking/percpu-rwsem: Use this_cpu_{inc,dec}() " tip-bot2 for Hou Tao
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