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Date:   Fri, 18 Sep 2020 15:26:35 +0200
From:   Jan Kara <jack@suse.cz>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     peterz@infradead.org, Jan Kara <jack@suse.cz>,
        Boaz Harrosh <boaz@plexistor.com>,
        Hou Tao <houtao1@huawei.com>, 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
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count
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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@suse.com>
SUSE Labs, CR