Re: [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention

[Vlastimil Babka]

On 1/4/19 1:50 PM, Mel Gorman wrote:
> Scanning on large machines can take a considerable length of time and
> eventually need to be rescheduled. This is treated as an abort event but
> that's not appropriate as the attempt is likely to be retried after making
> numerous checks and taking another cycle through the page allocator.
> This patch will check the need to reschedule if necessary but continue
> the scanning.
> 
> The main benefit is reduced scanning when compaction is taking a long time
> or the machine is over-saturated. It also avoids an unnecessary exit of
> compaction that ends up being retried by the page allocator in the outer
> loop.
> 
>                                         4.20.0                 4.20.0
>                               synccached-v2r15        noresched-v2r15
> Amean     fault-both-3      2655.55 (   0.00%)     2736.50 (  -3.05%)
> Amean     fault-both-5      4580.67 (   0.00%)     4133.70 (   9.76%)
> Amean     fault-both-7      5740.50 (   0.00%)     5738.61 (   0.03%)
> Amean     fault-both-12     9237.55 (   0.00%)     9392.82 (  -1.68%)
> Amean     fault-both-18    12899.51 (   0.00%)    13257.15 (  -2.77%)
> Amean     fault-both-24    16342.47 (   0.00%)    16859.44 (  -3.16%)
> Amean     fault-both-30    20394.26 (   0.00%)    16249.30 *  20.32%*
> Amean     fault-both-32    17450.76 (   0.00%)    14904.71 *  14.59%*

I always assumed that this was the main factor that (clumsily) limited THP fault
latencies. Seems like it's (no longer?) the case, or the lock contention
detection alone works as well.

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/compaction.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 1a41a2dbff24..75eb0d40d4d7 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -398,19 +398,11 @@ static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
>  	return true;
>  }
>  
> -/*
> - * Aside from avoiding lock contention, compaction also periodically checks
> - * need_resched() and records async compaction as contended if necessary.
> - */
> +/* Avoid soft-lockups due to long scan times */
>  static inline void compact_check_resched(struct compact_control *cc)
>  {
> -	/* async compaction aborts if contended */
> -	if (need_resched()) {
> -		if (cc->mode == MIGRATE_ASYNC)
> -			cc->contended = true;
> -
> +	if (need_resched())
>  		cond_resched();

Seems like plain "cond_resched()" is sufficient at this point, and probably
doesn't need a wrapper anymore.

> -	}
>  }
>  
>  /*
>