Re: [RFC] mm/vmscan.c: avoid possible long latency caused by too_many_isolated()

[Xing Zhengjun]

Hi Hillf,

On 4/30/2021 2:43 PM, Hillf Danton wrote:
> On Fri, 30 Apr 2021 13:33:57 +0800 Xing Zhengjun wrote:
> > I use my compaction test case to test it, 1/10 ratio can reproduce 100ms
> > sleep.
> >
> >   60) @ 103942.6 us |      shrink_node();
> >
> >   60) @ 103795.8 us |      shrink_node();
>
> Thanks for your test.
>
> In bid to cut the number of 100ms sleepers further down, add another place
> for them to nap by flushing lru cache before falling in sleep, instead of
> mulling why 50ms or 10ms is more adequate.
>
> Alternatively, and simpler IMHO, take a 5ms nap one time until !tmi.
>
> --- y/mm/vmscan.c
> +++ x/mm/vmscan.c
> @@ -118,6 +118,9 @@ struct scan_control {
>   	/* The file pages on the current node are dangerously low */
>   	unsigned int file_is_tiny:1;
>   
> +	unsigned int file_tmi:1; /* too many isolated */
> +	unsigned int anon_tmi:1;
> +
>   	/* Allocation order */
>   	s8 order;
>   
> @@ -2092,6 +2095,22 @@ static int current_may_throttle(void)
>   		bdi_write_congested(current->backing_dev_info);
>   }
>   
> +static void set_sc_tmi(struct scan_control *sc, bool file, int tmi)
> +{
> +	if (file)
> +		sc->file_tmi = tmi;
> +	else
> +		sc->anon_tmi = tmi;
> +}
> +
> +static bool is_sc_tmi(struct scan_control *sc, bool file)
> +{
> +	if (file)
> +		return sc->file_tmi != 0;
> +	else
> +		return sc->anon_tmi != 0;
> +}
> +
>   /*
>    * shrink_inactive_list() is a helper for shrink_node().  It returns the number
>    * of reclaimed pages
> @@ -2109,11 +2128,23 @@ shrink_inactive_list(unsigned long nr_to
>   	enum vm_event_item item;
>   	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>   	bool stalled = false;
> +	bool drained = false;
>   
>   	while (unlikely(too_many_isolated(pgdat, file, sc))) {
>   		if (stalled)
>   			return 0;
>   
> +		if (!is_sc_tmi(sc, file)) {
> +			set_sc_tmi(sc, file, 1);
> +			return 0;
> +		}
> +
> +		if (!drained) {
> +			drained = true;
> +			lru_add_drain_all();
> +			continue;
> +		}
> +
>   		/* wait a bit for the reclaimer. */
>   		msleep(100);
>   		stalled = true;
> @@ -2123,6 +2154,9 @@ shrink_inactive_list(unsigned long nr_to
>   			return SWAP_CLUSTER_MAX;
>   	}
>   
> +	if (is_sc_tmi(sc, file))
> +		set_sc_tmi(sc, file, 0);
> +
>   	lru_add_drain();
>   
>   	spin_lock_irq(&lruvec->lru_lock);

I tried the patch, it still can reproduce the 100ms sleep.

52) @ 103829.8 us |      shrink_lruvec();

--
Zhengjun Xing