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Date: Tue, 10 Nov 2009 14:42:20 +0900
From: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Spencer Candland <spencer@...ehost.com>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Oleg Nesterov <oleg@...hat.com>, sgruszka@...hat.com
Subject: Re: utime/stime decreasing on thread exit
Peter Zijlstra wrote:
> On Thu, 2009-11-05 at 14:24 +0900, Hidetoshi Seto wrote:
>
>> Problem [1]:
>> thread_group_cputime() vs exit
(snip)
>
> I just checked .22 and there we seem to hold p->sighand->siglock over
> the full task iteration. So we might as well revert back to that if
> people really mind counting things twice :-)
>
> FWIW getrusage() also takes siglock over the task iteration.
>
> Alternatively, we could try reading the sig->[us]time before doing the
> loop, but I guess that's still racy in that we can then miss someone
> altogether.
Right. So "before or after" will not be any help.
As you pointed there are some other functions taking siglock over the
task iteration, so making do_sys_times() do same will be acceptable.
In other words using many threads have risks, which might be solved in
future.
I agree that the following patch is right fix for this.
[PATCH 1/2] posix-cpu-timers: avoid do_sys_times() races with __exit_signal()
http://marc.info/?l=linux-kernel&m=124505545131145
>> Problem [2]:
>> use of task_s/utime()
>>
(snip)
>> [kernel/exit.c]
>> + sig->utime = cputime_add(sig->utime, task_utime(tsk));
>> + sig->stime = cputime_add(sig->stime, task_stime(tsk));
>>
>> While the thread_group_cputime() accumulates raw s/utime in do-while loop,
>> the signal struct accumulates adjusted s/utime of exited threads.
>>
>> I'm not sure how this adjustment works but applying the following patch
>> makes the result little bit better:
>>
>> :
>> times decreased : (436 741) (436 741) (437 744) (436 742) (436 742) (436 742)
>> times decreased : (454 792) (454 792) (455 794) (454 792) (454 792) (454 792)
>> times decreased : (503 941) (503 941) (504 943) (503 941) (503 941) (503 941)
>> :
>>
>> But still decreasing(or increasing) continues, because there is a problem [1]
>> at least.
>>
>> I think I couldn't handle this problem any more... Anybody can help?
>
> Stick in a few trace_printk()s and see what happens?
Nice idea.
I tried it and show the result in later of this mail.
>> Subject: [PATCH] thread_group_cputime() should use task_s/utime()
>>
>> The signal struct accumulates adjusted cputime of exited threads,
>> so thread_group_cputime() should use task_s/utime() instead of raw
>> task->s/utime, to accumulate adjusted cputime of live threads.
>>
>> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
>> ---
>> kernel/posix-cpu-timers.c | 4 ++--
>> 1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
>> index 5c9dc22..e065b8a 100644
>> --- a/kernel/posix-cpu-timers.c
>> +++ b/kernel/posix-cpu-timers.c
>> @@ -248,8 +248,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>>
>> t = tsk;
>> do {
>> - times->utime = cputime_add(times->utime, t->utime);
>> - times->stime = cputime_add(times->stime, t->stime);
>> + times->utime = cputime_add(times->utime, task_utime(t));
>> + times->stime = cputime_add(times->stime, task_stime(t));
>> times->sum_exec_runtime += t->se.sum_exec_runtime;
>>
>> t = next_thread(t);
>
> So what you're trying to say is that because __exit_signal() uses
> task_[usg]time() to accumulate sig->[usg]time, we should use it too in
> the loop over the live threads?
Right. Thank you for trying to understand.
>
> I'm thinking its the task_[usg]time() usage in __exit_signal() that's
> the issue.
It likely means reverting:
commit 49048622eae698e5c4ae61f7e71200f265ccc529
Author: Balbir Singh <balbir@...ux.vnet.ibm.com>
Date: Fri Sep 5 18:12:23 2008 +0200
sched: fix process time monotonicity
I'm not sure the reason why the task_[usg]time() have introduced, but
removing them would be one of solutions.
> I tried running the modified test.c on a current -tip kernel but could
> not observe the problem (dual-core opteron).
It might not happen if your box is quite fast (otherwise slow).
Changing loop in test.c (i.e. cycles in user-land) might help the
reproductivity...
Here is the result of additional test:
I put a trace_printk() in the __exit_signal(), to print tsk->s/utime,
task_s/utime() and tsk->se.sum_exec_rumtime.
(And tsk->prev_s/utime before calling task_s/utime())
<...>-2857 [006] 112.731732: release_task: (37 22)to(40 20), sxr 57480477, (0 0)
<...>-5077 [009] 526.272338: release_task: (0 27)to(10 20), sxr 27997019, (0 0)
<...>-4999 [009] 526.272396: release_task: (1 27)to(10 20), sxr 27967513, (0 0)
<...>-4992 [006] 526.328591: release_task: (2 34)to(10 30), sxr 35823013, (0 0)
<...>-5022 [012] 526.329183: release_task: (0 27)to(10 20), sxr 27761854, (0 0)
<...>-4996 [010] 526.329203: release_task: (3 38)to(10 30), sxr 39200207, (0 0)
... It clearly probes that there is the 3rd problem.
Problem [3]:
granularity of task_s/utime()
According to the git log, originally task_s/utime() were designed to
return clock_t but later changed to return cputime_t by following
commit:
commit efe567fc8281661524ffa75477a7c4ca9b466c63
Author: Christian Borntraeger <borntraeger@...ibm.com>
Date: Thu Aug 23 15:18:02 2007 +0200
It only changes the type of return value, but not the implementation.
As the result the granularity of task_s/utime() is still that of clock_t,
not that of cputime_t.
So using task_s/utime() in __exit_signal() makes values accumulated to the
signal struct to be rounded and coarse grained.
After applying a fix (will follow to this mail), return values are changed
to be fine grained:
<...>-5438 [006] 135.212289: release_task: (0 28)to(0 26), sxr 26648558, (0 0)
<...>-5402 [015] 135.213193: release_task: (0 27)to(0 26), sxr 26725886, (0 0)
<...>-5408 [011] 135.214172: release_task: (0 28)to(0 26), sxr 26607882, (0 0)
<...>-5419 [005] 135.214410: release_task: (1 27)to(1 25), sxr 26612615, (0 0)
<...>-5350 [009] 135.214937: release_task: (0 28)to(0 27), sxr 27028388, (0 0)
<...>-5443 [008] 135.216748: release_task: (0 28)to(0 26), sxr 26372691, (0 0)
But it cannot stop adjusted values become smaller than tsk->s/utime.
So some approach to solve problem [2] is required.
Thanks,
H.Seto
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