linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read

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Message-ID: <20200918132635.GI18920@quack2.suse.cz>
Date:   Fri, 18 Sep 2020 15:26:35 +0200
From:   Jan Kara <jack@...e.cz>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     peterz@...radead.org, Jan Kara <jack@...e.cz>,
        Boaz Harrosh <boaz@...xistor.com>,
        Hou Tao <houtao1@...wei.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count

On Fri 18-09-20 15:09:14, Oleg Nesterov wrote:
> On 09/18, Peter Zijlstra wrote:
> > > But again, do we really want this?
> >
> > I like the two counters better, avoids atomics entirely, some archs
> > hare horridly expensive atomics (*cough* power *cough*).
> 
> I meant... do we really want to introduce percpu_up_read_irqsafe() ?
> 
> Perhaps we can live with the fix from Hou? At least until we find a
> "real" performance regression.

I can say that for users of percpu rwsem in filesystems the cost of atomic
inc/dec is unlikely to matter. The lock hold times there are long enough
that it would be just lost in the noise.

For other stuff using them like get_online_cpus() or get_online_mems() I'm
not so sure...

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR