[PATCH] Fix pacct bug in multithreading case.
I noticed a bug on the process accounting facility. In multi-threading
process, some data would be recorded incorrectly when the group_leader dies
earlier than one or more threads. The attached patch fixes this problem.
See below. 'bugacct' is a test program that create a worker thread after 4
seconds sleeping, then the group_leader dies soon. The worker thread
consume CPU/Memory for 6 seconds, then exit. We can estimate 10 seconds as
etime and 6 seconds as stime + utime. This is a sample program which the
group_leader dies earlier than other threads.
The results of same binary execution on different kernel are below.
-- accounted records --------------------
| btime | utime | stime | etime | minflt | majflt | comm |
original | 13:16:40 | 0.00 | 0.00 | 6.10 | 171 | 0 | bugacct |
patched | 13:20:21 | 5.83 | 0.18 | 10.03 | 32776 | 0 | bugacct |
(*) bugacct allocates 128MB memory, thus 128MB / 4KB = 32768 of minflt is
appropriate.
-- Test results in original kernel ------
$ date; time -p ./bugacct
Tue Mar 28 13:16:36 JST 2006 <- But pacct said btime is 13:16:40
real 10.11 <- But pacct said etime is 6.10
user 5.96 <- But pacct said utime is 0.00
sys 0.14 <- But pacct said stime is 0.00
$
-- Test results in patched kernel -------
$ date; time -p ./bugacct
Tue Mar 28 13:20:21 JST 2006
real 10.04
user 5.83
sys 0.19
$
In the original 2.6.16 kernel, pacct records btime, utime, stime, etime and
minflt incorrectly. In my opinion, this problem is caused by an assumption
that group_leader dies last.
The following section calculates process running time for etime and btime.
But it means running time of the thread that dies last, not process. The
start_time of the first thread in the process (group_leader) should be
reduced from uptime to calculate etime and btime correctly.
---- do_acct_process() in kernel/acct.c:
/* calculate run_time in nsec*/
do_posix_clock_monotonic_gettime(&uptime);
run_time = (u64)uptime.tv_sec*NSEC_PER_SEC + uptime.tv_nsec;
run_time -= (u64)current->start_time.tv_sec*NSEC_PER_SEC
+ current->start_time.tv_nsec;
----
The following section calculates stime and utime of the process.
But it might count the utime and stime of the group_leader duplicatly
and ignore the utime and stime of the thread dies last, when one or
more threads remain after group_leader dead.
The ac_utime should be calculated as the sum of the signal->utime
and utime of the thread dies last. The ac_stime should be done also.
---- do_acct_process() in kernel/acct.c:
jiffies = cputime_to_jiffies(cputime_add(current->group_leader->utime,
current->signal->utime));
ac.ac_utime = encode_comp_t(jiffies_to_AHZ(jiffies));
jiffies = cputime_to_jiffies(cputime_add(current->group_leader->stime,
current->signal->stime));
ac.ac_stime = encode_comp_t(jiffies_to_AHZ(jiffies));
----
The part of the minflt/majflt calculation has same problem.
This patch solves those problems, I think.
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
1 file changed