Re: [LKP] [fs/locks] 83b381078b: will-it-scale.per_thread_ops -62.5% regression

From: NeilBrown
Date: Tue Nov 27 2018 - 18:20:46 EST

On Tue, Nov 27 2018, J. Bruce Fields wrote:

> Thanks for the report!

Yes, thanks. I thought I had replied to the previous report of a similar
problem, but I didn't actually send that email - oops.
Though the test is the same and the regression similar, this is a
different patch. The previous report identified
fs/locks: allow a lock request to block other requests
this one identifies
fs/locks: always delete_block after waiting.

Both cause blocked_lock_lock to be taken more often.

In one case is it due to locks_move_blocks(). That can probably be
optimised to skip the lock if list_empty(&fl->fl_blocked_requests).
I'd need to double-check, but I think that is safe to check without
locking.

This one causes locks_delete_blocks() to be called more often. We now
call it even if no waiting happened at all. I suspect we can test for
that and avoid it. I'll have a look.

>
> On Tue, Nov 27, 2018 at 02:01:02PM +0800, kernel test robot wrote:
>> FYI, we noticed a -62.5% regression of will-it-scale.per_thread_ops due to commit:
>>
>>
>> commit: 83b381078b5ecab098ebf6bc9548bb32af1dbf31 ("fs/locks: always delete_block after waiting.")
>> https://git.kernel.org/cgit/linux/kernel/git/jlayton/linux.git locks-next
>>
>> in testcase: will-it-scale
>> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
>> with following parameters:
>>
>> nr_task: 16
>> mode: thread
>> test: lock1
>
> So I guess it's doing this, uncontended file lock/unlock?:
>
> https://github.com/antonblanchard/will-it-scale/blob/master/tests/lock1.c
>
> Each thread is repeatedly locking and unlocking a file that is only used
> by that thread.

Thanks for identifying that Bruce.
This would certainly be a case where locks_delete_block() is now being
called when it wasn't before.


>
> By the way, what's the X-axis on these graphs? (Or the y-axis, for that
> matter?)

A key would help. I think the X-axis is number-of-threads. y-axis
might be ops-per-second ??.

Thanks,
NeilBrown


>
> --b.
>
>> will-it-scale.per_thread_ops
>>
>> 450000 +-+----------------------------------------------------------------+
>> | |
>> 400000 +-+ +..+.. .+..+.. .+..+..+...+..+..+.. +.. .+.. ..|
>> 350000 +-+ .. +. +. .. +. +..+ |
>> | + + + : |
>> 300000 +-+ : : |
>> 250000 +-+ : : |
>> | : : |
>> 200000 +-+ : : |
>> 150000 +-+ : : |
>> O O O O O O O O O O O O O O O O O :O: O O O O O
>> 100000 +-+ : : |
>> 50000 +-+ : : |
>> | : |
>> 0 +-+----------------------------------------------------------------+
>>
>>
>> will-it-scale.workload
>>
>> 7e+06 +-+-----------------------------------------------------------------+
>> | +...+.. .+..+..+ + +.. |
>> 6e+06 +-+ +..+.. .. .+..+..+. + + + .. ..|
>> | .. + +. + + + + +..+ |
>> 5e+06 +-++ + + : |
>> | : : |
>> 4e+06 +-+ : : |
>> | : : |
>> 3e+06 +-+ : : |
>> | O O : : O O |
>> 2e+06 O-+O O O O O O O O O O O O O O : O: O O O
>> | : : |
>> 1e+06 +-+ : : |
>> | : |
>> 0 +-+-----------------------------------------------------------------+
>>
>>
>> will-it-scale.time.user_time
>>
>> 250 +-+-------------------------------------------------------------------+
>> | .+.. .+.. +.. |
>> |.. +...+.. .+. .+...+..+..+. +.. +.. .. . ..|
>> 200 +-+ .. +. +. . .. + +..+ |
>> | + + + : |
>> | : : |
>> 150 +-+ : : |
>> | : : |
>> 100 +-+ : : |
>> | O O : : |
>> O O O O O O O O O O O O O O O :O: O O O O O
>> 50 +-+ : : |
>> | : : |
>> | : |
>> 0 +-+-------------------------------------------------------------------+
>>
>>
>> will-it-scale.time.system_time
>>
>> 5000 +-+------------------------------------------------------------------+
>> 4500 O-+O..O..O...O..O..O..O..O..O..O...O..O..O..O..O..O O O...O..O..O..O
>> | : : |
>> 4000 +-+ : : |
>> 3500 +-+ : : |
>> | : : |
>> 3000 +-+ : : |
>> 2500 +-+ : : |
>> 2000 +-+ : : |
>> | : : |
>> 1500 +-+ : : |
>> 1000 +-+ : : |
>> | : |
>> 500 +-+ : |
>> 0 +-+------------------------------------------------------------------+
>>
>>
>> [*] bisect-good sample
>> [O] bisect-bad sample

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