From: Jan Kara <jack@suse.cz>
To: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
linux-block@vger.kernel.org, kernel-team@fb.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH block 2/2] writeback, cgroup: inode_switch_wbs() shouldn't give up on wb_switch_rwsem trylock fail
Date: Thu, 15 Aug 2019 15:53:22 +0200 [thread overview]
Message-ID: <20190815135322.GI14313@quack2.suse.cz> (raw)
In-Reply-To: <20190802190813.GC136335@devbig004.ftw2.facebook.com>
On Fri 02-08-19 12:08:13, Tejun Heo wrote:
> As inode wb switching may make sync(2) miss some inodes, they're
> synchronized using wb_switch_rwsem so that no wb switching happens
> while sync(2) is in progress. In addition to synchronizing the actual
> switching, the rwsem is also used to prevent queueing new switch
> attempts while sync(2) is in progress. This is to avoid queueing too
> many instances while the rwsem is held by sync(2). Unfortunately,
> this is too agressive and can block wb switching for a long time if
> sync(2) is frequent.
>
> The goal is avoiding expolding the number of scheduled switches, not
> avoiding scheduling anything. Let's use wb_switch_rwsem only for
> synchronizing the actual switching and sync(2) and use
> isw_nr_in_flight instead for limiting the maximum number of scheduled
> switches. The limit is set to 1024 which should be more than enough
> while still avoiding extreme situations.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
Looks good to me. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>
Honza
> ---
> fs/fs-writeback.c | 17 +++++------------
> 1 file changed, 5 insertions(+), 12 deletions(-)
>
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -237,6 +237,7 @@ static void wb_wait_for_completion(struc
> /* if foreign slots >= 8, switch */
> #define WB_FRN_HIST_MAX_SLOTS (WB_FRN_HIST_THR_SLOTS / 2 + 1)
> /* one round can affect upto 5 slots */
> +#define WB_FRN_MAX_IN_FLIGHT 1024 /* don't queue too many concurrently */
>
> static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
> static struct workqueue_struct *isw_wq;
> @@ -489,18 +490,13 @@ static void inode_switch_wbs(struct inod
> if (inode->i_state & I_WB_SWITCH)
> return;
>
> - /*
> - * Avoid starting new switches while sync_inodes_sb() is in
> - * progress. Otherwise, if the down_write protected issue path
> - * blocks heavily, we might end up starting a large number of
> - * switches which will block on the rwsem.
> - */
> - if (!down_read_trylock(&bdi->wb_switch_rwsem))
> + /* avoid queueing a new switch if too many are already in flight */
> + if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
> return;
>
> isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
> if (!isw)
> - goto out_unlock;
> + return;
>
> /* find and pin the new wb */
> rcu_read_lock();
> @@ -534,15 +530,12 @@ static void inode_switch_wbs(struct inod
> call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
>
> atomic_inc(&isw_nr_in_flight);
> -
> - goto out_unlock;
> + return;
>
> out_free:
> if (isw->new_wb)
> wb_put(isw->new_wb);
> kfree(isw);
> -out_unlock:
> - up_read(&bdi->wb_switch_rwsem);
> }
>
> /**
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-08-15 13:53 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 19:07 [PATCH block 1/2] writeback, cgroup: Adjust WB_FRN_TIME_CUT_DIV to accelerate foreign inode switching Tejun Heo
2019-08-02 19:08 ` [PATCH block 2/2] writeback, cgroup: inode_switch_wbs() shouldn't give up on wb_switch_rwsem trylock fail Tejun Heo
2019-08-15 13:53 ` Jan Kara [this message]
2019-08-15 13:48 ` [PATCH block 1/2] writeback, cgroup: Adjust WB_FRN_TIME_CUT_DIV to accelerate foreign inode switching Jan Kara
2019-08-15 19:25 ` [PATCH v2 " Tejun Heo
2019-08-15 19:31 ` Jens Axboe
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