linux-kernel - Re: [PATCH 1/4] percpu_ref: introduce PERCPU_REF_ALLOW

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Message-ID: <20190509175244.GA27607@dennisz-mbp>
Date:   Thu, 9 May 2019 13:52:44 -0400
From:   Dennis Zhou <dennis@...nel.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Roman Gushchin <guro@...com>, Jens Axboe <axboe@...nel.dk>,
        Song Liu <songliubraving@...com>,
        Dennis Zhou <dennis@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 1/4] percpu_ref: introduce PERCPU_REF_ALLOW_REINIT flag

On Thu, May 09, 2019 at 09:53:43AM -0700, Tejun Heo wrote:
> On Tue, May 07, 2019 at 10:01:47AM -0700, Roman Gushchin wrote:
> > In most cases percpu reference counters are not switched to the
> > percpu mode after they reach the atomic mode. Some obvious exceptions
> > are reference counters which are initialized into the atomic
> > mode (using PERCPU_REF_INIT_ATOMIC and PERCPU_REF_INIT_DEAD flags),
> > and there are few other exceptions.
> > 
> > But in most cases there is no way back, and once the reference counter
> > is switched to the atomic mode, there is no reason to wait for
> > percpu_ref_exit() to release the percpu memory. Of course, the size
> > of a single counter is not so big, but because it can pin the whole
> > percpu block in memory, the memory footprint can be noticeable
> > (e.g. on my 32 CPUs machine a percpu block is 8Mb large).
> > 
> > To make releasing of the percpu memory as early as possible, let's
> > introduce the PERCPU_REF_ALLOW_REINIT flag with the following semantics:
> > it has to be set in order to switch a percpu reference counter to the
> > percpu mode after the initialization. PERCPU_REF_INIT_ATOMIC and
> > PERCPU_REF_INIT_DEAD flags will implicitly assume PERCPU_REF_ALLOW_REINIT.
> > 
> > This patch doesn't introduce any functional change to avoid any
> > regressions. It will be done later in the patchset after adjusting
> > all call sites, which are reviving percpu counters.
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> 
> For all patches in the series:
> 
>   Acked-by: Tejun Heo <tj@...rel.org>
> 
> Thanks.
> 
> -- 
> tejun

Great, I've applied this to for-5.3.

Thanks,
Dennis