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

[Lin Feng <linf@wangsu.com>]

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);