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[PATCH] Fix pacct bug in multithreading case.

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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>
This commit is contained in:
KaiGai Kohei 2006-03-31 02:30:45 -08:00 committed by Linus Torvalds
parent f79e2abb9b
commit bb231fe3a5
1 changed files with 6 additions and 6 deletions
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12
kernel/acct.c
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@ -449,8 +449,8 @@ static void do_acct_process(long exitcode, struct file *file)
/* 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;
run_time -= (u64)current->group_leader->start_time.tv_sec * NSEC_PER_SEC
+ current->group_leader->start_time.tv_nsec;
/* convert nsec -> AHZ */
elapsed = nsec_to_AHZ(run_time);
#if ACCT_VERSION==3
@ -469,10 +469,10 @@ static void do_acct_process(long exitcode, struct file *file)
#endif
do_div(elapsed, AHZ);
ac.ac_btime = xtime.tv_sec - elapsed;
jiffies = cputime_to_jiffies(cputime_add(current->group_leader->utime,
jiffies = cputime_to_jiffies(cputime_add(current->utime,
current->signal->utime));
ac.ac_utime = encode_comp_t(jiffies_to_AHZ(jiffies));
jiffies = cputime_to_jiffies(cputime_add(current->group_leader->stime,
jiffies = cputime_to_jiffies(cputime_add(current->stime,
current->signal->stime));
ac.ac_stime = encode_comp_t(jiffies_to_AHZ(jiffies));
/* we really need to bite the bullet and change layout */
@ -522,9 +522,9 @@ static void do_acct_process(long exitcode, struct file *file)
ac.ac_io = encode_comp_t(0 /* current->io_usage */); /* %% */
ac.ac_rw = encode_comp_t(ac.ac_io / 1024);
ac.ac_minflt = encode_comp_t(current->signal->min_flt +
current->group_leader->min_flt);
current->min_flt);
ac.ac_majflt = encode_comp_t(current->signal->maj_flt +
current->group_leader->maj_flt);
current->maj_flt);
ac.ac_swaps = encode_comp_t(0);
ac.ac_exitcode = exitcode;