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Date: Wed, 21 Jul 2021 16:35:05 -0600
From: Khalid Aziz <khalid.aziz@...cle.com>
To: Charan Teja Reddy <charante@...eaurora.org>,
akpm@...ux-foundation.org, vbabka@...e.cz, rientjes@...gle.com,
nigupta@...dia.com
Cc: vinmenon@...eaurora.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] mm: compaction: optimize proactive compaction
deferrals
On 7/21/21 6:13 AM, Charan Teja Reddy wrote:
> Vlastimil Babka figured out that when fragmentation score didn't go down
> across the proactive compaction i.e. when no progress is made, next wake
> up for proactive compaction is deferred for 1 <<
> COMPACT_MAX_DEFER_SHIFT, i.e. 64 times, with each wakeup interval of
> HPAGE_FRAG_CHECK_INTERVAL_MSEC(=500). In each of this wakeup, it just
> decrement 'proactive_defer' counter and goes sleep i.e. it is getting
> woken to just decrement a counter. The same deferral time can also
> achieved by simply doing the HPAGE_FRAG_CHECK_INTERVAL_MSEC <<
> COMPACT_MAX_DEFER_SHIFT thus unnecessary wakeup of kcompact thread is
> avoided thus also removes the need of 'proactive_defer' thread counter.
>
> Link: https://lore.kernel.org/linux-fsdevel/88abfdb6-2c13-b5a6-5b46-742d12d1c910@suse.cz/
> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>
> ---
> Changes in V1:
> o Removed the 'proactive_defer' thread counter by optimizing proactive
> o This is a resend as earlier it was clubbed with other changes posted
> at https://lore.kernel.org/patchwork/patch/1448789/
>
> mm/compaction.c | 29 +++++++++++++++++++----------
> 1 file changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 621508e..db00dbf 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2885,7 +2885,8 @@ static int kcompactd(void *p)
> {
> pg_data_t *pgdat = (pg_data_t *)p;
> struct task_struct *tsk = current;
> - unsigned int proactive_defer = 0;
> + long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC);
> + long timeout = default_timeout;
>
> const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>
> @@ -2902,23 +2903,30 @@ static int kcompactd(void *p)
>
> trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
> if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> - kcompactd_work_requested(pgdat),
> - msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
> + kcompactd_work_requested(pgdat), timeout)) {
>
> psi_memstall_enter(&pflags);
> kcompactd_do_work(pgdat);
> psi_memstall_leave(&pflags);
> + /*
> + * Reset the timeout value. The defer timeout by
> + * proactive compaction can effectively lost
> + * here but that is fine as the condition of the
> + * zone changed substantionally and carrying on
> + * with the previous defer is not useful.
> + */
> + timeout = default_timeout;
> continue;
> }
>
> - /* kcompactd wait timeout */
> + /*
> + * Start the proactive work with default timeout. Based
> + * on the fragmentation score, this timeout is updated.
> + */
> + timeout = default_timeout;
> if (should_proactive_compact_node(pgdat)) {
> unsigned int prev_score, score;
>
> - if (proactive_defer) {
> - proactive_defer--;
> - continue;
> - }
> prev_score = fragmentation_score_node(pgdat);
> proactive_compact_node(pgdat);
> score = fragmentation_score_node(pgdat);
> @@ -2926,8 +2934,9 @@ static int kcompactd(void *p)
> * Defer proactive compaction if the fragmentation
> * score did not go down i.e. no progress made.
> */
> - proactive_defer = score < prev_score ?
> - 0 : 1 << COMPACT_MAX_DEFER_SHIFT;
> + if (unlikely(score >= prev_score))
> + timeout =
> + default_timeout << COMPACT_MAX_DEFER_SHIFT;
> }
> }
>
>
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