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Date: Mon, 15 Mar 2021 18:31:00 +0800
From: Chao Yu <yuchao0@...wei.com>
To: Sahitya Tummala <stummala@...eaurora.org>
CC: Jaegeuk Kim <jaegeuk@...nel.org>,
<linux-f2fs-devel@...ts.sourceforge.net>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] f2fs: fix the discard thread sleep timeout under high
utilization
Hi Sahitya,
On 2021/3/15 17:45, Sahitya Tummala wrote:
> 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
I think so.
> 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;
> + }
Hmm.. how about cleaning up to configure discard policy in
__init_discard_policy()?
Something like:
---
fs/f2fs/segment.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 592927ccffa7..684463a70eb9 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1118,7 +1118,9 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
dpolicy->ordered = true;
if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL) {
dpolicy->granularity = 1;
- dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;
+ if (atomic_read(&SM_I(sbi)->dcc_info->discard_cmd_cnt))
+ dpolicy->max_interval =
+ DEF_MIN_DISCARD_ISSUE_TIME;
}
} else if (discard_type == DPOLICY_FORCE) {
dpolicy->min_interval = DEF_MIN_DISCARD_ISSUE_TIME;
@@ -1734,8 +1736,15 @@ static int issue_discard_thread(void *data)
set_freezable();
do {
- __init_discard_policy(sbi, &dpolicy, DPOLICY_BG,
- dcc->discard_granularity);
+ if (sbi->gc_mode == GC_URGENT_HIGH ||
+ !f2fs_available_free_memory(sbi, DISCARD_CACHE))
+ __init_discard_policy(sbi, &dpolicy, DPOLICY_FORCE, 1);
+ else
+ __init_discard_policy(sbi, &dpolicy, DPOLICY_BG,
+ dcc->discard_granularity);
+
+ if (!atomic_read(&dcc->discard_cmd_cnt))
+ wait_ms = dpolicy.max_interval;
wait_event_interruptible_timeout(*q,
kthread_should_stop() || freezing(current) ||
@@ -1762,10 +1771,6 @@ static int issue_discard_thread(void *data)
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);
-
sb_start_intwrite(sbi->sb);
issued = __issue_discard_cmd(sbi, &dpolicy);
--
2.29.2
Thoughts?
Thanks,
> +
> sb_start_intwrite(sbi->sb);
>
> issued = __issue_discard_cmd(sbi, &dpolicy);
>
> thanks,
> Sahitya.
>
>> Thanks,
>>
>>>
>>> Thanks,
>>> Sahitya.
>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
>>>>> ---
>>>>> 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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