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@xxxxxxxxx: Wake up from compaction context]
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
...
> 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? 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.
> +}
> +
> 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?
> long ret;
> DEFINE_WAIT(wait);
>
> @@ -1053,7 +1052,7 @@ void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page,
> READ_ONCE(pgdat->nr_reclaim_start);
>
> if (nr_written > SWAP_CLUSTER_MAX * nr_throttled)
> - wake_up_all(&pgdat->reclaim_wait);
> + wake_up_all(&pgdat->reclaim_wait[VMSCAN_THROTTLE_WRITEBACK]);
> }
>
> /* possible outcome of pageout() */
