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Date: Wed, 22 Sep 2021 07:27:31 +1000
From: "NeilBrown" <neilb@...e.de>
To: "Mel Gorman" <mgorman@...hsingularity.net>
Cc: "Linux-MM" <linux-mm@...ck.org>, "Theodore Ts'o" <tytso@....edu>,
"Andreas Dilger" <adilger.kernel@...ger.ca>,
"Darrick J . Wong" <djwong@...nel.org>,
"Matthew Wilcox" <willy@...radead.org>,
"Michal Hocko" <mhocko@...e.com>,
"Dave Chinner" <david@...morbit.com>,
"Rik van Riel" <riel@...riel.com>,
"Vlastimil Babka" <vbabka@...e.cz>,
"Johannes Weiner" <hannes@...xchg.org>,
"Jonathan Corbet" <corbet@....net>,
"Linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback
completes if congested
On Tue, 21 Sep 2021, Mel Gorman wrote:
> On Tue, Sep 21, 2021 at 09:19:07AM +1000, NeilBrown wrote:
> > On Mon, 20 Sep 2021, Mel Gorman wrote:
> > >
> > > +void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page);
> > > +static inline void acct_reclaim_writeback(struct page *page)
> > > +{
> > > + pg_data_t *pgdat = page_pgdat(page);
> > > +
> > > + if (atomic_read(&pgdat->nr_reclaim_throttled))
> > > + __acct_reclaim_writeback(pgdat, page);
> >
> > The first thing __acct_reclaim_writeback() does is repeat that
> > atomic_read().
> > Should we read it once and pass the value in to
> > __acct_reclaim_writeback(), or is that an unnecessary
> > micro-optimisation?
> >
>
> I think it's a micro-optimisation but I can still do it.
>
> >
> > > +/*
> > > + * Account for pages written if tasks are throttled waiting on dirty
> > > + * pages to clean. If enough pages have been cleaned since throttling
> > > + * started then wakeup the throttled tasks.
> > > + */
> > > +void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page)
> > > +{
> > > + unsigned long nr_written;
> > > + int nr_throttled = atomic_read(&pgdat->nr_reclaim_throttled);
> > > +
> > > + __inc_node_page_state(page, NR_THROTTLED_WRITTEN);
> > > + nr_written = node_page_state(pgdat, NR_THROTTLED_WRITTEN) -
> > > + READ_ONCE(pgdat->nr_reclaim_start);
> > > +
> > > + if (nr_written > SWAP_CLUSTER_MAX * nr_throttled)
> > > + wake_up_interruptible_all(&pgdat->reclaim_wait);
> >
> > A simple wake_up() could be used here. "interruptible" is only needed
> > if non-interruptible waiters should be left alone. "_all" is only needed
> > if there are some exclusive waiters. Neither of these apply, so I think
> > the simpler interface is best.
> >
>
> You're right.
>
> >
> > > +}
> > > +
> > > /* possible outcome of pageout() */
> > > typedef enum {
> > > /* failed to write page out, page is locked */
> > > @@ -1412,9 +1453,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
> > >
> > > /*
> > > * The number of dirty pages determines if a node is marked
> > > - * reclaim_congested which affects wait_iff_congested. kswapd
> > > - * will stall and start writing pages if the tail of the LRU
> > > - * is all dirty unqueued pages.
> > > + * reclaim_congested. kswapd will stall and start writing
> > > + * pages if the tail of the LRU is all dirty unqueued pages.
> > > */
> > > page_check_dirty_writeback(page, &dirty, &writeback);
> > > if (dirty || writeback)
> > > @@ -3180,19 +3220,20 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> > > * If kswapd scans pages marked for immediate
> > > * reclaim and under writeback (nr_immediate), it
> > > * implies that pages are cycling through the LRU
> > > - * faster than they are written so also forcibly stall.
> > > + * faster than they are written so forcibly stall
> > > + * until some pages complete writeback.
> > > */
> > > if (sc->nr.immediate)
> > > - congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > + reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
> > > }
> > >
> > > /*
> > > * Tag a node/memcg as congested if all the dirty pages
> > > * scanned were backed by a congested BDI and
> >
> > "congested BDI" doesn't mean anything any more. Is this a good time to
> > correct that comment.
> > This comment seems to refer to the test
> >
> > sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
> >
> > a few lines down. But nr.congested is set from nr_congested which
> > counts when inode_write_congested() is true - almost never - and when
> > "writeback and PageReclaim()".
> >
> > Is that last test the sign that we are cycling through the LRU to fast?
> > So the comment could become:
> >
> > Tag a node/memcg as congested if all the dirty page were
> > already marked for writeback and immediate reclaim (counted in
> > nr.congested).
> >
> > ??
> >
> > Patch seems to make sense to me, but I'm not expert in this area.
> >
>
> Comments updated.
>
> Diff on top looks like
>
> diff --git a/mm/internal.h b/mm/internal.h
> index e25b3686bfab..90764d646e02 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -34,13 +34,15 @@
>
> void page_writeback_init(void);
>
> -void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page);
> +void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page,
> + int nr_throttled);
> static inline void acct_reclaim_writeback(struct page *page)
> {
> pg_data_t *pgdat = page_pgdat(page);
> + int nr_throttled = atomic_read(&pgdat->nr_reclaim_throttled);
>
> - if (atomic_read(&pgdat->nr_reclaim_throttled))
> - __acct_reclaim_writeback(pgdat, page);
> + if (nr_throttled)
> + __acct_reclaim_writeback(pgdat, page, nr_throttled);
> }
>
> vm_fault_t do_swap_page(struct vm_fault *vmf);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b58ea0b13286..2dc17de91d32 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1034,10 +1034,10 @@ reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> * pages to clean. If enough pages have been cleaned since throttling
> * started then wakeup the throttled tasks.
> */
> -void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page)
> +void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page,
> + int nr_throttled)
> {
> unsigned long nr_written;
> - int nr_throttled = atomic_read(&pgdat->nr_reclaim_throttled);
>
> __inc_node_page_state(page, NR_THROTTLED_WRITTEN);
> nr_written = node_page_state(pgdat, NR_THROTTLED_WRITTEN) -
> @@ -3228,9 +3228,8 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> }
>
> /*
> - * Tag a node/memcg as congested if all the dirty pages
> - * scanned were backed by a congested BDI and
> - * non-kswapd tasks will stall on reclaim_throttle.
> + * Tag a node/memcg as congested if all the dirty pages were marked
> + * for writeback and immediate reclaim (counted in nr.congested).
> *
> * Legacy memcg will stall in page writeback so avoid forcibly
> * stalling in reclaim_throttle().
> @@ -3241,8 +3240,8 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> set_bit(LRUVEC_CONGESTED, &target_lruvec->flags);
>
> /*
> - * Stall direct reclaim for IO completions if underlying BDIs
> - * and node is congested. Allow kswapd to continue until it
> + * Stall direct reclaim for IO completions if the lruvec is
> + * node is congested. Allow kswapd to continue until it
> * starts encountering unqueued dirty pages or cycling through
> * the LRU too quickly.
> */
> @@ -4427,7 +4426,7 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
>
> trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, highest_zoneidx, order,
> gfp_flags);
> - wake_up_interruptible(&pgdat->kswapd_wait);
> + wake_up_all(&pgdat->kswapd_wait);
???
That isn't the wake_up that I pointed too.
Other changes look good - thanks.
NeilBrown
> }
>
> #ifdef CONFIG_HIBERNATION
>
>
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