Re: [RFC Patch] mm/vmscan.c: not inherit classzone_idx from previous reclaim

[Mel Gorman]

On Sun, Feb 09, 2020 at 03:41:45PM +0800, Wei Yang wrote:
> Before commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping
> prematurely due to mismatched classzone_idx"), classzone_idx could have
> two possibilities on a new loop based on whether there is a wakeup
> during reclaiming:
> 
>   * 0 if no wakeup
>   * the classzone_idx request by wakeup
> 
> As described in the changelog, this commit is willing to change the
> first case to (MAX_NR_ZONES - 1) to avoid some premature sleep. But it
> does not achieve the goal.
> 
> There are two versions of kswapd_classzone_idx() since this change:
> 
>   * commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping
>     prematurely due to mismatched classzone_idx")
>   * commit dffcac2cb88e ("mm/vmscan.c: prevent useless kswapd loops")
> 
> Both of them would return the classzone_idx we passed as the 2nd
> parameter when (pgdat->kswapd_classzone_idx == MAX_NR_ZONES). This
> means if there is no wakeup during reclaiming, we would use
> classzone_idx in previous round to sleep.
> 

This is somewhat intended.

> This patch fixes the logic by using (MAX_NR_ZONES - 1) for the first
> case.
> 

Ok, what is the user-visible impact that is fixed by this patch or is
this based on code review only? Please describe the test case exactly
and the before and after results. I ask because this area is a magnet for
regressions and intuitive ideas often lead to counter-intuitive results.

-- 
Mel Gorman
SUSE Labs