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Date: Thu, 14 Oct 2021 17:44:50 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Mel Gorman <mgorman@...hsingularity.net>
Cc: Linux-MM <linux-mm@...ck.org>, NeilBrown <neilb@...e.de>,
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>,
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 2/8] mm/vmscan: Throttle reclaim and compaction when too
may pages are isolated
On 10/14/21 13:56, Mel Gorman wrote:
> On Thu, Oct 14, 2021 at 10:06:25AM +0200, Vlastimil Babka wrote:
>> On 10/8/21 15:53, Mel Gorman wrote:
>> > Page reclaim throttles on congestion if too many parallel reclaim instances
>> > have isolated too many pages. This makes no sense, excessive parallelisation
>> > has nothing to do with writeback or congestion.
>> >
>> > This patch creates an additional workqueue to sleep on when too many
>> > pages are isolated. The throttled tasks are woken when the number
>> > of isolated pages is reduced or a timeout occurs. There may be
>> > some false positive wakeups for GFP_NOIO/GFP_NOFS callers but
>> > the tasks will throttle again if necessary.
>> >
>> > [shy828301@...il.com: Wake up from compaction context]
>> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
>>
>> ...
>>
>> > diff --git a/mm/internal.h b/mm/internal.h
>> > index 90764d646e02..06d0c376efcd 100644
>> > --- a/mm/internal.h
>> > +++ b/mm/internal.h
>> > @@ -45,6 +45,15 @@ static inline void acct_reclaim_writeback(struct page *page)
>> > __acct_reclaim_writeback(pgdat, page, nr_throttled);
>> > }
>> >
>> > +static inline void wake_throttle_isolated(pg_data_t *pgdat)
>> > +{
>> > + wait_queue_head_t *wqh;
>> > +
>> > + wqh = &pgdat->reclaim_wait[VMSCAN_THROTTLE_ISOLATED];
>> > + if (waitqueue_active(wqh))
>> > + wake_up_all(wqh);
>>
>> Again, would it be better to wake up just one task to prevent possible
>> thundering herd? We can assume that that task will call too_many_isolated()
>> eventually to wake up the next one?
>
> Same problem as the writeback throttling, there is no prioritsation of
> light vs heavy allocators.
>
>> Although it seems strange that
>> too_many_isolated() is the place where we detect the situation for wake up.
>> Simpler than to hook into NR_ISOLATED decrementing I guess.
>>
>
> Simplier but more costly. Every decrement would have to check
> too_many_isolated(). I think the cost of that is too high given that the
> VMSCAN_THROTTLE_ISOLATED is relatively hard to trigger and the minority
> of throttling events.
Agreed.
>> > +}
>> > +
>> > vm_fault_t do_swap_page(struct vm_fault *vmf);
>> >
>> > void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>> ...
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1006,11 +1006,10 @@ static void handle_write_error(struct address_space *mapping,
>> > unlock_page(page);
>> > }
>> >
>> > -static void
>> > -reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
>> > +void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
>> > long timeout)
>> > {
>> > - wait_queue_head_t *wqh = &pgdat->reclaim_wait;
>> > + wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
>>
>> It seems weird that later in this function we increase nr_reclaim_throttled
>> without distinguishing the reason, so effectively throttling for isolated
>> pages will trigger acct_reclaim_writeback() doing the NR_THROTTLED_WRITTEN
>> counting, although it's not related at all? Maybe either have separate
>> nr_reclaim_throttled counters per vmscan_throttle_state (if counter of
>> isolated is useful, I haven't seen the rest of series yet), or count only
>> VMSCAN_THROTTLE_WRITEBACK tasks?
>>
>
> Very good point, it would be more appropriate to only count the
> writeback reason.
>
> Diff on top is below. It'll cause minor conflicts later in the series.
Looks good, for the updated version:
Acked-by: Vlastimil Babka <vbabka@...e.cz>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index ca65d6a64bdd..58a25d42c31c 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -849,7 +849,7 @@ typedef struct pglist_data {
> wait_queue_head_t kswapd_wait;
> wait_queue_head_t pfmemalloc_wait;
> wait_queue_head_t reclaim_wait[NR_VMSCAN_THROTTLE];
> - atomic_t nr_reclaim_throttled; /* nr of throtted tasks */
> + atomic_t nr_writeback_throttled;/* nr of writeback-throttled tasks */
> unsigned long nr_reclaim_start; /* nr pages written while throttled
> * when throttling started. */
> struct task_struct *kswapd; /* Protected by
> diff --git a/mm/internal.h b/mm/internal.h
> index 06d0c376efcd..3461a1055975 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -39,7 +39,7 @@ 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);
> - int nr_throttled = atomic_read(&pgdat->nr_reclaim_throttled);
> + int nr_throttled = atomic_read(&pgdat->nr_writeback_throttled);
>
> if (nr_throttled)
> __acct_reclaim_writeback(pgdat, page, nr_throttled);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6e198bbbd86a..29434d4fc1c7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1011,6 +1011,7 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> {
> wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
> long ret;
> + bool acct_writeback = (reason == VMSCAN_THROTTLE_WRITEBACK);
> DEFINE_WAIT(wait);
>
> /*
> @@ -1022,7 +1023,8 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> current->flags & (PF_IO_WORKER|PF_KTHREAD))
> return;
>
> - if (atomic_inc_return(&pgdat->nr_reclaim_throttled) == 1) {
> + if (acct_writeback &&
> + atomic_inc_return(&pgdat->nr_writeback_throttled) == 1) {
> WRITE_ONCE(pgdat->nr_reclaim_start,
> node_page_state(pgdat, NR_THROTTLED_WRITTEN));
> }
> @@ -1030,7 +1032,9 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
> ret = schedule_timeout(timeout);
> finish_wait(wqh, &wait);
> - atomic_dec(&pgdat->nr_reclaim_throttled);
> +
> + if (acct_writeback)
> + atomic_dec(&pgdat->nr_writeback_throttled);
>
> trace_mm_vmscan_throttled(pgdat->node_id, jiffies_to_usecs(timeout),
> jiffies_to_usecs(timeout - ret),
> @@ -4349,7 +4353,7 @@ static int kswapd(void *p)
>
> WRITE_ONCE(pgdat->kswapd_order, 0);
> WRITE_ONCE(pgdat->kswapd_highest_zoneidx, MAX_NR_ZONES);
> - atomic_set(&pgdat->nr_reclaim_throttled, 0);
> + atomic_set(&pgdat->nr_writeback_throttled, 0);
> for ( ; ; ) {
> bool ret;
>
>
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