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 12:57:58 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     DaeRo Lee <skseofh@...il.com>
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

On Thu, Jan 06, 2022 at 09:03:34PM +0900, DaeRo Lee wrote:
> > > @@ -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:)
> 

Stupid typo on my part.

> > @@ -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!
> 

Feel free to pick it up, rerun your tests to ensure it's behaving as
expected and resend! Include something in the changelog about user-visible
effects if any (or a note saying that it reduces unnecssary overhead)
and resend with me added to the cc.

> > > @@ -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;;

No need to be sorry, it happens :)

Thanks!

-- 
Mel Gorman
SUSE Labs