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Subject: Re: [RFC] mm/vmscan.c: avoid possible long latency caused by
 too_many_isolated()
To:     Yu Zhao <yuzhao@google.com>,
        Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, ying.huang@intel.com,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@suse.com>, wfg@mail.ustc.edu.cn
References: <20210416023536.168632-1-zhengjun.xing@linux.intel.com>
 <7b7a1c09-3d16-e199-15d2-ccea906d4a66@linux.intel.com>
 <YIGuvh70JbE1Cx4U@google.com>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <a085478d-5118-cdff-c611-1649fce7a650@linux.intel.com>
Date:   Thu, 22 Apr 2021 13:17:37 -0700
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On 4/22/21 10:13 AM, Yu Zhao wrote:

> @@ -3302,6 +3252,7 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
>  unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  				gfp_t gfp_mask, nodemask_t *nodemask)
>  {
> +	int nr_cpus;
>  	unsigned long nr_reclaimed;
>  	struct scan_control sc = {
>  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
> @@ -3334,8 +3285,17 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  	set_task_reclaim_state(current, &sc.reclaim_state);
>  	trace_mm_vmscan_direct_reclaim_begin(order, sc.gfp_mask);
>  
> +	nr_cpus = current_is_kswapd() ? 0 : num_online_cpus();
> +	while (nr_cpus && !atomic_add_unless(&pgdat->nr_reclaimers, 1, nr_cpus)) {
> +		if (schedule_timeout_killable(HZ / 10))

100 msec seems like a long time to wait.  The original code in shrink_inactive_list
choose 100 msec sleep because the sleep happens only once in the while loop and 100 msec was
used to check for stalling.  In this case the loop can go on for a while and the 
#reclaimers can go down below the sooner than 100 msec. Seems like it should be checked
more often.

Tim

> +			return SWAP_CLUSTER_MAX;
> +	}
> +
>  	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
>  
> +	if (nr_cpus)
> +		atomic_dec(&pgdat->nr_reclaimers);
> +
>  	trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
>  	set_task_reclaim_state(current, NULL);
>