linux-kernel - [PATCH] blk-iocost: fix false positive lagging

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Message-Id: <20220526133554.21079-1-zhouchengming@bytedance.com>
Date:   Thu, 26 May 2022 21:35:54 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     tj@...nel.org, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH] blk-iocost: fix false positive lagging

I found many false positive lagging during iocost test.

Since iocg->vtime will be advanced to (vnow - margins.target)
in hweight_after_donation(), which called throw away excess,
the iocg->done_vtime will also be advanced that much.

       period_at_vtime  <--period_vtime-->  vnow
              |                              |
  --------------------------------------------------->
        |<--->|
     margins.target
        |->
  vtime, done_vtime

If that iocg has some inflight io when vnow, but its done_vtime
is before period_at_vtime, ioc_timer_fn() will think it has
lagging io, even these io maybe issued just before now.

This patch change the condition to check if vdone is before
(period_at_vtime - margins.target) instead of period_at_vtime.

But there is another problem that this patch doesn't fix.
Since vtime will be advanced, we can't check if vtime is
after (vnow - MAX_LAGGING_PERIODS * period_vtime) to tell
whether this iocg pin lagging for too long.

Maybe we can add lagging_periods in iocg to record how many
periods this iocg pin lagging, but I don't know when to clean it.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 block/blk-iocost.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 33a11ba971ea..42e301b7527b 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -2259,7 +2259,7 @@ static void ioc_timer_fn(struct timer_list *timer)
 		    time_after64(vtime, vdone) &&
 		    time_after64(vtime, now.vnow -
 				 MAX_LAGGING_PERIODS * period_vtime) &&
-		    time_before64(vdone, now.vnow - period_vtime))
+		    time_before64(vdone, ioc->period_at_vtime - ioc->margins.target))
 			nr_lagging++;

 		/*
-- 
2.36.1