From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-xfs@vger.kernel.org, dm-devel@redhat.com,
Mikulas Patocka <mpatocka@redhat.com>,
Jens Axboe <axboe@kernel.dk>, NeilBrown <neilb@suse.de>
Subject: Re: [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio
Date: Thu, 25 Jun 2020 14:42:34 +0200 [thread overview]
Message-ID: <20200625124234.GM1320@dhcp22.suse.cz> (raw)
In-Reply-To: <20200625123418.GB7703@casper.infradead.org>
On Thu 25-06-20 13:34:18, Matthew Wilcox wrote:
> On Thu, Jun 25, 2020 at 02:22:39PM +0200, Michal Hocko wrote:
> > On Thu 25-06-20 12:31:17, Matthew Wilcox wrote:
> > > We're short on PF_* flags, so make memalloc_noio its own bit where we
> > > have plenty of space.
> >
> > I do not mind moving that outside of the PF_* space. Unless I
> > misremember all flags in this space were intented to be set only on the
> > current which rules out any RMW races and therefore they can be
> > lockless. I am not sure this holds for the bitfield you are adding this
> > to. At least in_memstall seem to be set on external task as well. But
> > this would require double checking. Maybe that is not really intended or
> > just a bug.
>
> I was going from the comment:
>
> /* Unserialized, strictly 'current' */
> (which you can't see from the context of the diff, but is above the block)
>
> The situation with ->flags is a little more ambiguous:
>
> /*
> * Only the _current_ task can read/write to tsk->flags, but other
> * tasks can access tsk->flags in readonly mode for example
> * with tsk_used_math (like during threaded core dumping).
> * There is however an exception to this rule during ptrace
> * or during fork: the ptracer task is allowed to write to the
> * child->flags of its traced child (same goes for fork, the parent
> * can write to the child->flags), because we're guaranteed the
> * child is not running and in turn not changing child->flags
> * at the same time the parent does it.
> */
OK, I have obviously missed that.
> but it wasn't unsafe to use the PF_ flags in the way that you were.
> It's just crowded.
>
> If in_memstall is set on other tasks, then it should be moved to the
> PFA flags, which there are plenty of.
>
> But a quick grep shows it only being read on other tasks and always
> set on current:
>
> kernel/sched/psi.c: *flags = current->in_memstall;
> kernel/sched/psi.c: * in_memstall setting & accounting needs to be atomic wrt
> kernel/sched/psi.c: current->in_memstall = 1;
> kernel/sched/psi.c: * in_memstall clearing & accounting needs to be atomic wrt
> kernel/sched/psi.c: current->in_memstall = 0;
> kernel/sched/psi.c: if (task->in_memstall)
Have a look at cgroup_move_task. So I believe this is something to be
fixed but independent on your change.
Feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>
> kernel/sched/stats.h: if (p->in_memstall)
> kernel/sched/stats.h: if (p->in_memstall)
> kernel/sched/stats.h: if (unlikely(p->in_iowait || p->in_memstall)) {
> kernel/sched/stats.h: if (p->in_memstall)
> kernel/sched/stats.h: if (unlikely(rq->curr->in_memstall))
>
> so I think everything is fine.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-06-25 12:42 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-25 11:31 [PATCH 0/6] Overhaul memalloc_no* Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio Matthew Wilcox (Oracle)
2020-06-25 12:22 ` Michal Hocko
2020-06-25 12:34 ` Matthew Wilcox
2020-06-25 12:42 ` Michal Hocko [this message]
2020-06-25 11:31 ` [PATCH 2/6] mm: Add become_kswapd and restore_kswapd Matthew Wilcox (Oracle)
2020-06-25 12:31 ` Michal Hocko
2020-06-25 11:31 ` [PATCH 3/6] xfs: Convert to memalloc_nofs_save Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 4/6] mm: Replace PF_MEMALLOC_NOFS with memalloc_nofs Matthew Wilcox (Oracle)
2020-06-25 13:35 ` Michal Hocko
2020-06-25 11:31 ` [PATCH 5/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_nocma Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 6/6] mm: Add memalloc_nowait Matthew Wilcox (Oracle)
2020-06-25 12:40 ` Michal Hocko
2020-06-25 13:10 ` Matthew Wilcox
2020-06-25 13:34 ` Michal Hocko
2020-06-25 19:05 ` kernel test robot
2020-06-25 23:51 ` kernel test robot
2020-06-29 5:08 ` Mike Rapoport
2020-06-29 12:18 ` Matthew Wilcox
2020-06-29 12:52 ` Michal Hocko
2020-06-29 13:45 ` Mike Rapoport
2020-06-29 21:28 ` Matthew Wilcox
2020-06-30 6:34 ` Michal Hocko
2020-07-01 4:12 ` Matthew Wilcox
2020-07-01 5:53 ` Michal Hocko
2020-07-01 7:04 ` Mike Rapoport
2020-09-24 0:39 ` Mike Snitzer
2020-09-24 1:10 ` Matthew Wilcox
2020-10-23 14:49 ` [dm-devel] " Daniel Vetter
2020-06-25 18:48 ` [PATCH 0/6] Overhaul memalloc_no* Darrick J. Wong
2020-06-25 20:34 ` Matthew Wilcox
2020-06-25 20:36 ` Michal Hocko
2020-06-25 20:40 ` Matthew Wilcox
2020-06-26 15:02 ` Mikulas Patocka
2020-06-26 23:08 ` Dave Chinner
2020-06-27 13:09 ` Mikulas Patocka
2020-06-29 0:35 ` Dave Chinner
2020-06-29 13:43 ` Mikulas Patocka
2020-06-29 22:34 ` Dave Chinner
2020-07-03 14:26 ` [PATCH] dm-bufio: do cleanup from a workqueue Mikulas Patocka
2020-06-29 8:22 ` [PATCH 0/6] Overhaul memalloc_no* Michal Hocko
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=20200625124234.GM1320@dhcp22.suse.cz \
--to=mhocko@kernel.org \
--cc=axboe@kernel.dk \
--cc=dm-devel@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-xfs@vger.kernel.org \
--cc=mpatocka@redhat.com \
--cc=neilb@suse.de \
--cc=willy@infradead.org \
/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).