linux-kernel - Re: [PATCH v2 3/3] mm/slub: Use percpu partial free counter

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Message-ID: <20210303201649.GF2723601@casper.infradead.org>
Date:   Wed, 3 Mar 2021 20:16:49 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Lameter <cl@...two.de>
Cc:     Xunlei Pang <xlpang@...ux.alibaba.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Wen Yang <wenyang@...ux.alibaba.com>,
        Roman Gushchin <guro@...com>, Pekka Enberg <penberg@...il.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v2 3/3] mm/slub: Use percpu partial free counter

On Wed, Mar 03, 2021 at 08:55:48PM +0100, Christoph Lameter wrote:
> On Wed, 3 Mar 2021, Matthew Wilcox wrote:
> 
> > > Can this be allocated in an interrupt context?
> > >
> > > And I am not sure how local_t relates to that? Percpu counters can be used
> > > in an interrupt context without the overhead of the address calculations
> > > that are required by a local_t.
> >
> > As I understand the patch, this counts the number of partially free slabs.
> > So if we start to free an object from a completely full slab in process
> > context, as "load x, add one to x, store x" and take an interrupt
> > between loading x and adding one to x, that interrupt handler might
> > free a different object from another completely full slab.  that would
> > also load the same x, add one to it and store x, but then the process
> > context would add one to the old x, overwriting the updated value from
> > interrupt context.
> 
> this_cpu operations are "atomic" vs. preemption but on some platforms not
> vs interrupts. That could be an issue in kmem_cache_free(). This would
> need a modification to the relevant this_cpu ops so that interrupts are
> disabled on those platforms.

Hmmmm ... re-reading the documentation, it says that this_cpu_x is
atomic against interrupts:

These operations can be used without worrying about
preemption and interrupts::
[...]
        this_cpu_add(pcp, val)
        this_cpu_inc(pcp)
...