From 633a2abb9e1cd5c95f3b600f4b2c12cce22ae4a0 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 2 Sep 2021 14:53:04 -0700 Subject: writeback: track number of inodes under writeback Patch series "writeback: Fix bandwidth estimates", v4. Fix estimate of writeback throughput when device is not fully busy doing writeback. Michael Stapelberg has reported that such workload (e.g. generated by linking) tends to push estimated throughput down to 0 and as a result writeback on the device is practically stalled. The first three patches fix the reported issue, the remaining two patches are unrelated cleanups of problems I've noticed when reading the code. This patch (of 4): Track number of inodes under writeback for each bdi_writeback structure. We will use this to decide whether wb does any IO and so we can estimate its writeback throughput. In principle we could use number of pages under writeback (WB_WRITEBACK counter) for this however normal percpu counter reads are too inaccurate for our purposes and summing the counter is too expensive. Link: https://lkml.kernel.org/r/20210713104519.16394-1-jack@suse.cz Link: https://lkml.kernel.org/r/20210713104716.22868-1-jack@suse.cz Signed-off-by: Jan Kara Cc: Wu Fengguang Cc: Michael Stapelberg Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/backing-dev-defs.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux/backing-dev-defs.h') diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index 1d7edad9914f..06fb8e13f6bc 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -116,6 +116,7 @@ struct bdi_writeback { struct list_head b_dirty_time; /* time stamps are dirty */ spinlock_t list_lock; /* protects the b_* lists */ + atomic_t writeback_inodes; /* number of inodes under writeback */ struct percpu_counter stat[NR_WB_STAT_ITEMS]; unsigned long congested; /* WB_[a]sync_congested flags */ -- cgit From 45a2966fd64147518dc5bca25f447bd0fb5359ac Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 2 Sep 2021 14:53:09 -0700 Subject: writeback: fix bandwidth estimate for spiky workload Michael Stapelberg has reported that for workload with short big spikes of writes (GCC linker seem to trigger this frequently) the write throughput is heavily underestimated and tends to steadily sink until it reaches zero. This has rather bad impact on writeback throttling (causing stalls). The problem is that writeback throughput estimate gets updated at most once per 200 ms. One update happens early after we submit pages for writeback (at that point writeout of only small fraction of pages is completed and thus observed throughput is tiny). Next update happens only during the next write spike (updates happen only from inode writeback and dirty throttling code) and if that is more than 1s after previous spike, we decide system was idle and just ignore whatever was written until this moment. Fix the problem by making sure writeback throughput estimate is also updated shortly after writeback completes to get reasonable estimate of throughput for spiky workloads. [jack@suse.cz: avoid division by 0 in wb_update_dirty_ratelimit()] Link: https://lore.kernel.org/lkml/20210617095309.3542373-1-stapelberg+linux@google.com Link: https://lkml.kernel.org/r/20210713104716.22868-3-jack@suse.cz Signed-off-by: Jan Kara Reported-by: Michael Stapelberg Tested-by: Michael Stapelberg Cc: Wu Fengguang Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/backing-dev-defs.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux/backing-dev-defs.h') diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index 06fb8e13f6bc..33207004cfde 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -143,6 +143,7 @@ struct bdi_writeback { spinlock_t work_lock; /* protects work_list & dwork scheduling */ struct list_head work_list; struct delayed_work dwork; /* work item used for writeback */ + struct delayed_work bw_dwork; /* work item used for bandwidth estimate */ unsigned long dirty_sleep; /* last wait */ -- cgit