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

[Sahitya Tummala]

Hi Chao,

On Mon, Mar 15, 2021 at 04:10:22PM +0800, Chao Yu wrote:
> Hi Sahitya,
> 
> On 2021/3/15 15:46, Sahitya Tummala wrote:
> >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.
> 
> Copied, thanks for the explanation.
> 
> But there is another case which can cause this issue in the case of
> disk util < 80%.
> 
> wait_ms = DEF_MIN_DISCARD_ISSUE_TIME;
> 
> do {
> 	wait_event_interruptible_timeout(, wait_ms);
> 
> 	...
> 
> 	if (!atomic_read(&dcc->discard_cmd_cnt))
> [1] new statement
> 		continue;
> 
> } while();
> 
> Then the loop will wakeup whenever 50ms timeout.
> 
Yes, only for a short period of time i.e., until the first discard command
is issued. Once a discard is issued, it will use 
wait_ms = dpolicy.max_interval;

> So, to avoid this case, shouldn't we reset wait_ms to dpolicy.max_interval
> at [1]?
> 
Yes, we can add that to cover the above case. 

> Meanwhile, how about relocating discard_cmd_cnt check after
> __init_discard_policy(DPOLICY_FORCE)? and olny set .max_interval to
> DEF_MAX_DISCARD_ISSUE_TIME if there is no discard command, and keep
> .granularity to 1?
>

There is not need to change .granularity, right? It will be controlled
as per utilization as it is done today. Only max_interval and wait_ms
needs to be updated. Does this look good?

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index d7076796..958ad1e 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1772,13 +1772,16 @@ static int issue_discard_thread(void *data)
                        wait_ms = dpolicy.max_interval;
                        continue;
                }
-               if (!atomic_read(&dcc->discard_cmd_cnt))
-                       continue;
-
                if (sbi->gc_mode == GC_URGENT_HIGH ||
                        !f2fs_available_free_memory(sbi, DISCARD_CACHE))
                        __init_discard_policy(sbi, &dpolicy, DPOLICY_FORCE, 1);

+               if (!atomic_read(&dcc->discard_cmd_cnt)) {
+                       dpolicy.max_interval = DEF_MAX_DISCARD_ISSUE_TIME;
+                       wait_ms = dpolicy.max_interval;
+                       continue;
+               }
+
                sb_start_intwrite(sbi->sb);

                issued = __issue_discard_cmd(sbi, &dpolicy);

thanks,
Sahitya.

> Thanks,
> 
> >
> >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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