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

Xing Zhengjun
2021-04-30T05:34:07Z

Hi Hillf,

On 4/22/2021 6:23 PM, Hillf Danton wrote:
> Hi Zhengjun
>
> On Thu, 22 Apr 2021 16:36:19 +0800 Zhengjun Xing wrote:
>> In the system with very few file pages (nr_active_file +
>> nr_inactive_file < 100), it is easy to reproduce "nr_isolated_file >
>> nr_inactive_file", then too_many_isolated return true,
>> shrink_inactive_list enter "msleep(100)", the long latency will happen.
>
> We should skip reclaiming page cache in this case.
>>
>> The test case to reproduce it is very simple: allocate many huge
>> pages(near the DRAM size), then do free, repeat the same operation many
>> times.
>> In the test case, the system with very few file pages (nr_active_file +
>> nr_inactive_file < 100), I have dumpped the numbers of
>> active/inactive/isolated file pages during the whole test(see in the
>> attachments) , in shrink_inactive_list "too_many_isolated" is very easy
>> to return true, then enter "msleep(100)",in "too_many_isolated"
>> sc->gfp_mask is 0x342cca ("_GFP_IO" and "__GFP_FS" is masked) , it is
>> also very easy to enter “inactive >>=3”, then “isolated > inactive” will
>> be true.
>>
>> So I have a proposal to set a threshold number for the total file pages
>> to ignore the system with very few file pages, and then bypass the 100ms
>> sleep.
>> It is hard to set a perfect number for the threshold, so I just give an
>> example of "256" for it.
>
> Another option seems like we take a nap at the second time of lru tmi
> with some allocators in your case served without the 100ms delay.
>
> +++ 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;
>
> @@ -1905,6 +1908,21 @@ static int current_may_throttle(void)
> bdi_write_congested(current->backing_dev_info);
> }
>
> +static void update_sc_tmi(struct scan_control *sc, bool file, int set)
> +{
> + if (file)
> + sc->file_tmi = set;
> + else
> + sc->anon_tmi = set;
> +}
> +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
> @@ -1927,6 +1945,11 @@ shrink_inactive_list(unsigned long nr_to
> if (stalled)
> return 0;
>
> + if (!is_sc_tmi(sc, file)) {
> + update_sc_tmi(sc, file, 1);
> + return 0;
> + }
> +
> /* wait a bit for the reclaimer. */
> msleep(100);
> stalled = true;
> @@ -1936,6 +1959,9 @@ shrink_inactive_list(unsigned long nr_to
> return SWAP_CLUSTER_MAX;
> }
>
> + if (is_sc_tmi(sc, file))
> + update_sc_tmi(sc, file, 0);
> +
> lru_add_drain();
>
> spin_lock_irq(&lruvec->lru_lock);
>

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();

--
Zhengjun Xing