Re: [PATCH 4/8] mm: Only drain per-cpu pagevecs once per pagevec usage

Vlastimil Babka
2017-10-19T09:13:16Z

On 10/18/2017 09:59 AM, Mel Gorman wrote:
> When a pagevec is initialised on the stack, it is generally used multiple
> times over a range of pages, looking up entries and then releasing them.
> On each pagevec_release, the per-cpu deferred LRU pagevecs are drained
> on the grounds the page being released may be on those queues and the
> pages may be cache hot. In many cases only the first drain is necessary
> as it's unlikely that the range of pages being walked is racing against
> LRU addition. Even if there is such a race, the impact is marginal where
> as constantly redraining the lru pagevecs costs.

Right, the drain is only to a local cpu, not all of them, so that kind
of "racing" shouldn't be even possible.

> This patch ensures that pagevec is only drained once in a given lifecycle
> without increasing the cache footprint of the pagevec structure. Only

Well, strictly speaking it does prevent decreasing the cache footprint
by removing the 'cold' field later :)

> sparsetruncate tiny is shown here as large files have many exceptional
> entries and calls pagecache_release less frequently.
>
> sparsetruncate (tiny)
> 4.14.0-rc4 4.14.0-rc4
> batchshadow-v1r1 onedrain-v1r1
> Min Time 141.00 ( 0.00%) 141.00 ( 0.00%)
> 1st-qrtle Time 142.00 ( 0.00%) 142.00 ( 0.00%)
> 2nd-qrtle Time 142.00 ( 0.00%) 142.00 ( 0.00%)
> 3rd-qrtle Time 143.00 ( 0.00%) 143.00 ( 0.00%)
> Max-90% Time 144.00 ( 0.00%) 144.00 ( 0.00%)
> Max-95% Time 146.00 ( 0.00%) 145.00 ( 0.68%)
> Max-99% Time 198.00 ( 0.00%) 194.00 ( 2.02%)
> Max Time 254.00 ( 0.00%) 208.00 ( 18.11%)
> Amean Time 145.12 ( 0.00%) 144.30 ( 0.56%)
> Stddev Time 12.74 ( 0.00%) 9.62 ( 24.49%)
> Coeff Time 8.78 ( 0.00%) 6.67 ( 24.06%)
> Best99%Amean Time 144.29 ( 0.00%) 143.82 ( 0.32%)
> Best95%Amean Time 142.68 ( 0.00%) 142.31 ( 0.26%)
> Best90%Amean Time 142.52 ( 0.00%) 142.19 ( 0.24%)
> Best75%Amean Time 142.26 ( 0.00%) 141.98 ( 0.20%)
> Best50%Amean Time 141.90 ( 0.00%) 141.71 ( 0.13%)
> Best25%Amean Time 141.80 ( 0.00%) 141.43 ( 0.26%)
>
> The impact on bonnie is marginal and within the noise because a significant
> percentage of the file being truncated has been reclaimed and consists of
> shadow entries which reduce the hotness of the pagevec_release path.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> include/linux/pagevec.h | 4 +++-
> mm/swap.c | 5 ++++-
> 2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/pagevec.h b/include/linux/pagevec.h
> index 4dcd5506f1ed..4231979be982 100644
> --- a/include/linux/pagevec.h
> +++ b/include/linux/pagevec.h
> @@ -16,7 +16,8 @@ struct address_space;
>
> struct pagevec {
> unsigned long nr;
> - unsigned long cold;
> + bool cold;
> + bool drained;

'drained' sounds a bit misleading to me, I would expect it to refer to
*this* pagevec. What about e.g. "lru_drained"?

Thanks

> struct page *pages[PAGEVEC_SIZE];
> };
>
> @@ -45,6 +46,7 @@ static inline void pagevec_init(struct pagevec *pvec, int cold)
> {
> pvec->nr = 0;
> pvec->cold = cold;
> + pvec->drained = false;
> }
>
> static inline void pagevec_reinit(struct pagevec *pvec)
> diff --git a/mm/swap.c b/mm/swap.c
> index a77d68f2c1b6..31bd9d8a5db7 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -833,7 +833,10 @@ EXPORT_SYMBOL(release_pages);
> */
> void __pagevec_release(struct pagevec *pvec)
> {
> - lru_add_drain();
> + if (!pvec->drained) {
> + lru_add_drain();
> + pvec->drained = true;
> + }
> release_pages(pvec->pages, pagevec_count(pvec), pvec->cold);
> pagevec_reinit(pvec);
> }
>