Re: [PATCH] f2fs: fix the discard thread sleep timeout under high utilization

[Sahitya Tummala]

Hi Chao,

On Mon, Mar 15, 2021 at 02:12:44PM +0800, Chao Yu wrote:
> Sahitya,
> 
> On 2021/3/15 12:56, Sahitya Tummala wrote:
> >When f2fs is heavily utilized over 80%, the current discard policy
> >sets the max sleep timeout of discard thread as 50ms
> >(DEF_MIN_DISCARD_ISSUE_TIME). But this is set even when there are
> >no pending discard commands to be issued. This results into
> >unnecessary frequent and periodic wake ups of the discard thread.
> >This patch adds check for pending  discard commands in addition
> >to heavy utilization condition to prevent those wake ups.
> 
> Could this commit fix your issue?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git/commit/?h=dev&id=43f8c47ea7d59c7b2270835f1d7c4618a1ea27b6
> 
I don't think it will help because we are changing the max timeout of the
dpolicy itself in __init_discard_policy() when util > 80% as below -  

dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;

And issue_discard_thread() uses this value as wait_ms, when there
are no more pending discard commands to be issued.
<snip>
                } else {
                        wait_ms = dpolicy.max_interval;
                }
<snip>

The new patch posted above is not changing anything related to the  max_interval.
Hence, I think it won't help the uncessary wakeup problem I am trying to solve
for this condition - util > 80% and when there are no pending discards.

Please let me know if i am missing something.

Thanks,
Sahitya.

> Thanks,
> 
> >
> >Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
> >---
> >  fs/f2fs/segment.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> >diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >index dced46c..df30220 100644
> >--- a/fs/f2fs/segment.c
> >+++ b/fs/f2fs/segment.c
> >@@ -1112,6 +1112,8 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
> >  				struct discard_policy *dpolicy,
> >  				int discard_type, unsigned int granularity)
> >  {
> >+	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
> >+
> >  	/* common policy */
> >  	dpolicy->type = discard_type;
> >  	dpolicy->sync = true;
> >@@ -1129,7 +1131,8 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
> >  		dpolicy->io_aware = true;
> >  		dpolicy->sync = false;
> >  		dpolicy->ordered = true;
> >-		if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL) {
> >+		if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL &&
> >+				atomic_read(&dcc->discard_cmd_cnt)) {
> >  			dpolicy->granularity = 1;
> >  			dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;
> >  		}
> >

-- 
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