Messages in this thread | | | Subject | Re: [PATCH] [RFC] vmscan.c: add a sysctl entry for controlling memory reclaim IO congestion_wait length | From | Lin Feng <> | Date | Thu, 19 Sep 2019 15:46:11 +0800 |
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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);
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