linux-kernel - Re: [RFC 08/16] vfio/type1: Cache locked_vm to ease mmap

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Date:   Thu, 6 Jan 2022 08:34:56 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Alexander Duyck <alexanderduyck@...com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Segall <bsegall@...gle.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Josh Triplett <josh@...htriplett.org>,
        Michal Hocko <mhocko@...e.com>, Nico Pache <npache@...hat.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Steve Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-mm@...ck.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [RFC 08/16] vfio/type1: Cache locked_vm to ease mmap_lock
 contention

On Wed, Jan 05, 2022 at 08:17:08PM -0500, Daniel Jordan wrote:
> On Wed, Jan 05, 2022 at 08:53:39PM -0400, Jason Gunthorpe wrote:
> > On Wed, Jan 05, 2022 at 07:46:48PM -0500, Daniel Jordan wrote:
> > > padata threads hold mmap_lock as reader for the majority of their
> > > runtime in order to call pin_user_pages_remote(), but they also
> > > periodically take mmap_lock as writer for short periods to adjust
> > > mm->locked_vm, hurting parallelism.
> > > 
> > > Alleviate the write-side contention with a per-thread cache of locked_vm
> > > which allows taking mmap_lock as writer far less frequently.
> > > 
> > > Failure to refill the cache due to insufficient locked_vm will not cause
> > > the entire pinning operation to error out.  This avoids spurious failure
> > > in case some pinned pages aren't accounted to locked_vm.
> > > 
> > > Cache size is limited to provide some protection in the unlikely event
> > > of a concurrent locked_vm accounting operation in the same address space
> > > needlessly failing in case the cache takes more locked_vm than it needs.
> > 
> > Why not just do the pinned page accounting once at the start? Why does
> > it have to be done incrementally?
> 
> Yeah, good question.  I tried doing it that way recently and it did
> improve performance a bit, but I thought it wasn't enough of a gain to
> justify how it overaccounted by the size of the entire pin.

Why would it over account?

Jason