All of lore.kernel.org
 help / color / mirror / Atom feed
From: "NeilBrown" <neilb@suse.de>
To: "Mel Gorman" <mgorman@techsingularity.net>
Cc: "Linux-MM" <linux-mm@kvack.org>, "Theodore Ts'o" <tytso@mit.edu>,
	"Andreas Dilger" <adilger.kernel@dilger.ca>,
	"Darrick J . Wong" <djwong@kernel.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michal Hocko" <mhocko@suse.com>,
	"Dave Chinner" <david@fromorbit.com>,
	"Rik van Riel" <riel@surriel.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
	"LKML" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback completes if congested
Date: Wed, 22 Sep 2021 07:40:59 +1000	[thread overview]
Message-ID: <163226045956.21861.7998898955979000139@noble.neil.brown.name> (raw)
In-Reply-To: <20210921105831.GO3959@techsingularity.net>

On Tue, 21 Sep 2021, Mel Gorman wrote:
> On Tue, Sep 21, 2021 at 10:13:17AM +1000, NeilBrown wrote:
> > On Mon, 20 Sep 2021, Mel Gorman wrote:
> > > -long wait_iff_congested(int sync, long timeout)
> > > -{
> > > -	long ret;
> > > -	unsigned long start = jiffies;
> > > -	DEFINE_WAIT(wait);
> > > -	wait_queue_head_t *wqh = &congestion_wqh[sync];
> > > -
> > > -	/*
> > > -	 * If there is no congestion, yield if necessary instead
> > > -	 * of sleeping on the congestion queue
> > > -	 */
> > > -	if (atomic_read(&nr_wb_congested[sync]) == 0) {
> > > -		cond_resched();
> > > -
> > > -		/* In case we scheduled, work out time remaining */
> > > -		ret = timeout - (jiffies - start);
> > > -		if (ret < 0)
> > > -			ret = 0;
> > > -
> > > -		goto out;
> > > -	}
> > > -
> > > -	/* Sleep until uncongested or a write happens */
> > > -	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
> > 
> > Uninterruptible wait.
> > 
> > ....
> > > +static void
> > > +reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> > > +							long timeout)
> > > +{
> > > +	wait_queue_head_t *wqh = &pgdat->reclaim_wait;
> > > +	unsigned long start = jiffies;
> > > +	long ret;
> > > +	DEFINE_WAIT(wait);
> > > +
> > > +	atomic_inc(&pgdat->nr_reclaim_throttled);
> > > +	WRITE_ONCE(pgdat->nr_reclaim_start,
> > > +		 node_page_state(pgdat, NR_THROTTLED_WRITTEN));
> > > +
> > > +	prepare_to_wait(wqh, &wait, TASK_INTERRUPTIBLE);
> > 
> > Interruptible wait.
> > 
> > Why the change?  I think these waits really need to be TASK_UNINTERRUPTIBLE.
> > 
> 
> Because from mm/ context, I saw no reason why the task *should* be
> uninterruptible. It's waiting on other tasks to complete IO and it is not
> protecting device state, filesystem state or anything else. If it gets
> a signal, it's safe to wake up, particularly if that signal is KILL and
> the context is a direct reclaimer.

I disagree.  An Interruptible sleep only makes sense if the "was
interrupted" status can propagate up to user-space (or to some in-kernel
handler that will clear the signal).
In particular, if reclaim_throttle() is called in a loop (which it is),
and if that loop doesn't check for signal_pending (which it doesn't),
then the next time around the loop after receiving a signal, it won't
sleep at all.  That would be bad.

In general, if you don't return an error, then you probably shouldn't
sleep Interruptible.

I notice that tasks sleep on kswapd_wait as TASK_INTERRUPTIBLE, but they
don't have any signal handling.  I suspect this isn't actually a defect
because I suspect that is it not even possible to SIGKILL kswapd.  But
the code seems misleading.  I guess I should write a patch.

Unless reclaim knows to abort completely on a signal (__GFP_KILLABLE
???) this must be an UNINTERRUPTIBLE wait.

Thanks,
NeilBrown

> 
> The original TASK_UNINTERRUPTIBLE is almost certainly a copy&paste from
> congestion_wait which may be called because a filesystem operation must
> complete before it can return to userspace so a signal waking it up is
> pointless.
> 
> -- 
> Mel Gorman
> SUSE Labs
> 
> 

  reply	other threads:[~2021-09-21 21:41 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-20  8:54 [RFC PATCH 0/5] Remove dependency on congestion_wait in mm/ Mel Gorman
2021-09-20  8:54 ` [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback completes if congested Mel Gorman
2021-09-20 23:19   ` NeilBrown
2021-09-21 11:12     ` Mel Gorman
2021-09-21 21:27       ` NeilBrown
2021-09-21  0:13   ` NeilBrown
2021-09-21 10:58     ` Mel Gorman
2021-09-21 21:40       ` NeilBrown [this message]
2021-09-22  6:04       ` Dave Chinner
2021-09-22  8:03         ` Mel Gorman
2021-09-22 12:16   ` Hillf Danton
2021-09-22 14:13     ` Mel Gorman
2021-09-20  8:54 ` [PATCH 2/5] mm/vmscan: Throttle reclaim and compaction when too may pages are isolated Mel Gorman
2021-09-20 23:27   ` NeilBrown
2021-09-21 11:03     ` Mel Gorman
2021-09-21 18:45   ` Yang Shi
2021-09-21 18:45     ` Yang Shi
2021-09-22  8:11     ` Mel Gorman
2021-09-20  8:54 ` [PATCH 3/5] mm/vmscan: Throttle reclaim when no progress is being made Mel Gorman
2021-09-20 23:31   ` NeilBrown
2021-09-21 11:16     ` Mel Gorman
2021-09-21 21:46       ` NeilBrown
2021-09-22  9:21         ` Mel Gorman
2021-09-20  8:54 ` [PATCH 4/5] mm/writeback: Throttle based on page writeback instead of congestion Mel Gorman
2021-09-20  8:54 ` [PATCH 5/5] mm/page_alloc: Remove the throttling logic from the page allocator Mel Gorman
2021-09-20 11:42 ` [RFC PATCH 0/5] Remove dependency on congestion_wait in mm/ Matthew Wilcox
2021-09-20 12:50   ` Mel Gorman
2021-09-20 14:11     ` David Sterba
2021-09-21 11:18       ` Mel Gorman
2021-09-20 19:51   ` Mel Gorman
2021-09-21 20:46 ` Dave Chinner
2021-09-22 17:52   ` Mel Gorman
2021-09-29 10:09 [PATCH 0/5] Remove dependency on congestion_wait in mm/ v2 Mel Gorman
2021-09-29 10:09 ` [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback completes if congested Mel Gorman

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=163226045956.21861.7998898955979000139@noble.neil.brown.name \
    --to=neilb@suse.de \
    --cc=adilger.kernel@dilger.ca \
    --cc=corbet@lwn.net \
    --cc=david@fromorbit.com \
    --cc=djwong@kernel.org \
    --cc=hannes@cmpxchg.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mgorman@techsingularity.net \
    --cc=mhocko@suse.com \
    --cc=riel@surriel.com \
    --cc=tytso@mit.edu \
    --cc=vbabka@suse.cz \
    --cc=willy@infradead.org \
    --subject='Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback completes if congested' \
    /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

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.