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From: Oleg Nesterov <oleg@redhat.com>
To: Boaz Harrosh <boaz@plexistor.com>
Cc: Hou Tao <houtao1@huawei.com>,
	peterz@infradead.org, Ingo Molnar <mingo@redhat.com>,
	Will Deacon <will@kernel.org>, 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 14:01:33 +0200	[thread overview]
Message-ID: <20200917120132.GA5602@redhat.com> (raw)
In-Reply-To: <ddd5d732-06da-f8f2-ba4a-686c58297e47@plexistor.com>

On 09/17, Boaz Harrosh wrote:
>
> On 16/09/2020 15:32, Hou Tao wrote:
> <>
> >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 believe perhaps Peter Z's suggestion of an additional
> percpu_down_read_irqsafe() API and let only those in IRQ users pay the
> penalty.
>
> Peter Z wrote:
> >My leading alternative was adding: percpu_down_read_irqsafe() /
> >percpu_up_read_irqsafe(), which use local_irq_save() instead of
> >preempt_disable().

This means that __sb_start/end_write() and probably more users in fs/super.c
will have to use this API, not good.

IIUC, file_end_write() was never IRQ safe (at least if !CONFIG_SMP), even
before 8129ed2964 ("change sb_writers to use percpu_rw_semaphore"), but this
doesn't matter...

Perhaps we can change aio.c, io_uring.c and fs/overlayfs/file.c to avoid
file_end_write() in IRQ context, but I am not sure it's worth the trouble.

Oleg.


  reply	other threads:[~2020-09-17 12:12 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
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 [this message]
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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