Re: [PATCH] [RFC] vmscan.c: add a sysctl entry for controlling memory reclaim IO congestion_wait length

Lin Feng
2019-09-19T07:46:35Z

On 9/19/19 11:49, Matthew Wilcox wrote:
> On Thu, Sep 19, 2019 at 10:33:10AM +0800, Lin Feng wrote:
>> On 9/18/19 20:33, Michal Hocko wrote:
>>> I absolutely agree here. From you changelog it is also not clear what is
>>> the underlying problem. Both congestion_wait and wait_iff_congested
>>> should wake up early if the congestion is handled. Is this not the case?
>>
>> For now I don't know why, codes seem should work as you said, maybe I need to
>> trace more of the internals.
>> But weird thing is that once I set the people-disliked-tunable iowait
>> drop down instantly, this is contradictory to the code design.
>
> Yes, this is quite strange. If setting a smaller timeout makes a
> difference, that indicates we're not waking up soon enough. I see
> two possibilities; one is that a wakeup is missing somewhere -- ie the
> conditions under which we call clear_wb_congested() are wrong. Or we
> need to wake up sooner.
>
> Umm. We have clear_wb_congested() called from exactly one spot --
> clear_bdi_congested(). That is only called from:
>
> drivers/block/pktcdvd.c
> fs/ceph/addr.c
> fs/fuse/control.c
> fs/fuse/dev.c
> fs/nfs/write.c
>
> Jens, is something supposed to be calling clear_bdi_congested() in the
> block layer? blk_clear_congested() used to exist until October 29th
> last year. Or is something else supposed to be waking up tasks that
> are sleeping on congestion?
>

IIUC it looks like after commit a1ce35fa49852db60fc6e268038530be533c5b15,
besides those *.c places as you mentioned above, vmscan codes will always
wait as long as 100ms and nobody wakes them up.

here:
1964 while (unlikely(too_many_isolated(pgdat, file, sc))) {
1965 if (stalled)
1966 return 0;
1967
1968 /* wait a bit for the reclaimer. */
>1969 msleep(100);
1970 stalled = true;
1971
1972 /* We are about to die and free our memory. Return now. */
1973 if (fatal_signal_pending(current))
1974 return SWAP_CLUSTER_MAX;
1975 }

and here:
2784 /*
2785 * If kswapd scans pages marked marked for immediate
2786 * reclaim and under writeback (nr_immediate), it
2787 * implies that pages are cycling through the LRU
2788 * faster than they are written so also forcibly stall.
2789 */
2790 if (sc->nr.immediate)
>2791 congestion_wait(BLK_RW_ASYNC, HZ/10);
2792 }

except here, codes where set_bdi_congested will clear_bdi_congested at proper time,
exactly the source files you mentioned above, so it's OK.
2808 if (!sc->hibernation_mode && !current_is_kswapd() &&
2809 current_may_throttle() && pgdat_memcg_congested(pgdat, root))
2810 wait_iff_congested(BLK_RW_ASYNC, HZ/10);