linux-kernel - Re: [PATCH] mm/vmscan.c: no need to double-check if free pages are under high-watermark

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Date:   Thu, 6 Jan 2022 21:03:34 +0900
From:   DaeRo Lee <skseofh@...il.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: no need to double-check if free pages are
 under high-watermark

> > @@ -4355,7 +4355,7 @@ static enum zone_type kswapd_highest_zoneidx(pg_data_t *pgdat,
> >  static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
> >                               unsigned int highest_zoneidx)
> >  {
> > -     long remaining = 0;
> > +     long remaining = ~0;
> >       DEFINE_WAIT(wait);
> >
> >       if (freezing(current) || kthread_should_stop())
>
> While this does avoid calling prepare_kswapd_sleep() twice if the pgdat
> is balanced on the first try, it then does not restore the vmstat
> thresholds and doesn't call schedul() for kswapd to go to sleep.

I intended not to call prepare_kswapd_sleep() twice when the pgdat is NOT
balanced on the first try:)

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 700434db5735..40784693c840 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4355,7 +4355,8 @@ static enum zone_type kswapd_highest_zoneidx(pg_data_t *pgdat,
>  static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
>                                 unsigned int highest_zoneidx)
>  {
> -       long remaining = 0;
> +       long remaining;
> +       bool balanced;
>         DEFINE_WAIT(wait);
>
>         if (freezing(current) || kthread_should_stop())
> @@ -4370,7 +4371,8 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
>          * eligible zone balanced that it's also unlikely that compaction will
>          * succeed.
>          */
> -       if (prepare_kswapd_sleep(pgdat, reclaim_order, highest_zoneidx)) {
> +       balanced = prepare_kswapd_sleep(pgdat, reclaim_order, highest_zoneidx);
> +       if (balanced) {
>                 /*
>                  * Compaction records what page blocks it recently failed to
>                  * isolate pages from and skips them in the future scanning.
> @@ -4387,6 +4389,10 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
>
>                 remaining = schedule_timeout(HZ/10);
>
> +               /* Is pgdat balanced after a short sleep? */
> +               balanced = prepare_kswapd_sleep(pgdat, reclaim_order,
> +                                                       highest_zoneidx);
> +
>                 /*
>                  * If woken prematurely then reset kswapd_highest_zoneidx and
>                  * order. The values will either be from a wakeup request or
> @@ -4406,11 +4412,11 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
>         }
>
>         /*
> -        * After a short sleep, check if it was a premature sleep. If not, then
> -        * go fully to sleep until explicitly woken up.
> +        * If balanced to the high watermark, restore vmstat thresholds and
> +        * kswapd goes to sleep. If kswapd remains awake, account whether
> +        * the low or high watermark was hit quickly.
>          */
> -       if (!remaining &&
> -           prepare_kswapd_sleep(pgdat, reclaim_order, highest_zoneidx)) {
> +       if (balanced) {
>                 trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
>
>                 /*

But, I think what you did is more readable and nice.
Thanks!

> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 700434db5735..1217ecec5bbb 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4331,7 +4331,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
> >       /*
> >        * Return the order kswapd stopped reclaiming at as
> >        * prepare_kswapd_sleep() takes it into account. If another caller
> > -      * entered the allocator slow path while kswapd was awake, order will
> > +      * entered the allqocator slow path while kswapd was awake, order will
> >        * remain at the higher level.
> >        */
> >       return sc.order;
>
> This hunk just adds a typo, drop it.

Sorry about that;;