Re: [vserver] Kernel Thread's accounted to a User Context

From: Eric Keller <ekeller_at_Princeton.EDU>
Date: Sat 15 Dec 2007 - 15:35:30 GMT
Message-ID: <4763F442.1070000@princeton.edu>

As an update... I added counters to vx_schedule() (which gets called by
schedule()) and vx_need_resched() (which gets called by scheduler_tick()).

What I found is that a process does indeed run many ticks from when
vx_need_resched() gets called and runs out of tokens and sets
TIF_NEED_RESCHED to when schedule gets called to pick a new task. Since
tokens don't go below zero, and since I guess enough time has passed
from when the task was flagged to when vx_schedule() does get called,
the process gets some more tokens and never gets put on hold.

I added my counters to /proc/virtual/<context>/sched, and here's a
stripped down

[#] more /proc/virtual/777/sched
FillRate: 2,1
Interval: 40,8
TokensMin: 4
TokensMax: 200
PrioBias: 0
cpu 0: 364(usr) 1074(sys) 561(hold) 5226(tokentime) 0(idle) R- 0 4 200
2/40 1/8 0 0
     hold 0 zero_tok(k) 732 zero_tok(u) 0 vx_sched(k) 64 vx_sched(u) 343
     consume(k) 323 consume(u) 368 slice_zero 791

[#] more /proc/virtual/777/sched
FillRate: 2,1
Interval: 40,8
TokensMin: 4
TokensMax: 200
PrioBias: 0
cpu 0: 364(usr) 1353(sys) 561(hold) 5260(tokentime) 0(idle) R- 0 4 200
2/40 1/8 0 0
     hold 0 zero_tok(k) 977 zero_tok(u) 0 vx_sched(k) 81 vx_sched(u) 343
     consume(k) 357 consume(u) 368 slice_zero 1053

The first line (where it says cpu 0) is what vserver currently prints
out (with me adding text in parentheses).
The second line:
* hold: how many times the code vx_hold task gets called
* zero_tok: how many times vx_need_resched (and therefore
scheduler_tick) gets called while the token count is 0 (the k in
parentheses means a kernel thread, u means user thread)
* vx_sched: how many times vx_schedule gets called (again with k for
kernel and u for user)
The third line:
* consume: how many times _vx_consume_token gets called (again with k
for kernel and u for user)
* zero_slice: how many times vx_need_resched returns true (i.e. (slice==0)).

So, from this one can see that 279 sys ticks, 245 zero token ticks,
vx_schedule was called 17 times, 34 tokens were consumed. Note that the
245 zero token ticks + 34 tokens consumed = 279 sys ticks.

When I run just a user process, there are no zero_tok ticks and it does
indeed go on hold.

Any idea on how to make it so that the task immediately holds (is it bad
to call schedule() from scheduler_tick())?

Thanks,
Eric

       
Received on Sat Dec 15 15:35:47 2007

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