From: Christoph Lameter <cl@gentwo.de>
To: Matthew Wilcox <willy@infradead.org>
Cc: Xunlei Pang <xlpang@linux.alibaba.com>,
Vlastimil Babka <vbabka@suse.cz>,
Wen Yang <wenyang@linux.alibaba.com>,
Roman Gushchin <guro@fb.com>, Pekka Enberg <penberg@gmail.com>,
Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
David Rientjes <rientjes@google.com>,
linux-kernel@vger.kernel.org,
"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH v2 3/3] mm/slub: Use percpu partial free counter
Date: Wed, 3 Mar 2021 20:55:48 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.22.394.2103032032070.897408@gentwo.de> (raw)
In-Reply-To: <20210303193038.GE2723601@casper.infradead.org>
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.
Like this_cpu_inc_irq() or so?
> it's not the likeliest of races, and i don't know how important it is
> that these counters remain accurate. but using a local_t instead of
> a percpu long would fix the problem. i don't know why you think that
> a local_t needs "address calculations". perhaps you've misremembered
> what a local_t is?
local_t does not include the address arithmetic that the this_cpu
operation can implicitly perform on x86 f.e. with an segment register or
maybe another register on another platform thereby avoiding the need to
disable preemption or interrupts.
Therefore a manual calculation of the target address for a local_t
operation needs to be done beforehand which usually means disabling
interrupts and/or preemption for the code segment. Otherwise we may end up
on a different processor due to scheduler or other interruptions and use
the percpu counter value of a different processor which could be racy.
next prev parent reply other threads:[~2021-03-03 19:56 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-10 12:17 [PATCH v2 0/3] mm/slub: Fix count_partial() problem Xunlei Pang
2020-08-10 12:17 ` [PATCH v2 1/3] mm/slub: Introduce two counters for partial objects Xunlei Pang
2020-08-10 12:17 ` [PATCH v2 2/3] mm/slub: Get rid of count_partial() Xunlei Pang
2020-08-10 12:17 ` [PATCH v2 3/3] mm/slub: Use percpu partial free counter Xunlei Pang
2021-03-02 5:56 ` Shu Ming
2021-03-02 9:14 ` Christoph Lameter
2021-03-03 13:46 ` Xunlei Pang
2021-03-03 14:26 ` Matthew Wilcox
2021-03-03 19:15 ` Christoph Lameter
2021-03-03 19:30 ` Matthew Wilcox
2021-03-03 19:55 ` Christoph Lameter [this message]
2021-03-03 20:16 ` Matthew Wilcox
2020-08-20 14:02 ` [PATCH v2 0/3] mm/slub: Fix count_partial() problem Pekka Enberg
2020-08-24 10:04 ` xunlei
2021-03-01 10:31 ` Shu Ming
2021-03-03 13:34 ` Xunlei Pang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.DEB.2.22.394.2103032032070.897408@gentwo.de \
--to=cl@gentwo.de \
--cc=guro@fb.com \
--cc=khlebnikov@yandex-team.ru \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=penberg@gmail.com \
--cc=rientjes@google.com \
--cc=vbabka@suse.cz \
--cc=wenyang@linux.alibaba.com \
--cc=willy@infradead.org \
--cc=xlpang@linux.alibaba.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).