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SubjectRe: [PATCH] ftrace: Don't use tracing_record_cmdline() in workqueue tracer
Date
> 
> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
>
> > Currently, /sys/kernel/debug/tracing/trace_stat/workqueues can
> > display wrong and strang thread name.
> >
> > Why?
> >
> > Currently, ftrace has
> > tracing_record_cmdline()/trace_find_cmdline() convinience
> > function. they implement task->comm string cache. it can avoid
> > unnecessary memcpy overhead. and workqueue tracer use it.
> >
> > However, in general, any trace stastics feature shouldn't use
> > tracing_record_cmdline(). A trace stastics can display very
> > old process. then comm cache can return wrong string because
> > recent process override the cache.
> >
> > Fortunately, workqueue trace gerantee to live displayed
> > process. Then, we can search comm string from pid at display
> > time.
>
> Applied, thanks!
>
> We might need to improve the comm-cache - i've seen frequent
> artifacts due to it. Displaying <...> is _far_ better than an
> outright misleading string displayed.
>
> I.e. the cache should be improved to be properly coherent with
> reality.
>
> Ingo

Doh!
My last mail's cc list didn't include lkml. that's unintensional silly
my mistake. very sorry.

and Yes, memcpy(buf, task->comm, 16) mean two movq instruction.
that is worthless for caching. I think we can remove this cache completely.


and, I find my last patch has one race window. fixing below.

================================
Subject: [PATCH] tracing: Don't use tracing_record_cmdline() in workqueue tracer fix

last "Don't use tracing_record_cmdline() in workqueue tracer" patch have
a race window.

find_task_by_vpid() require task_list_lock(). but the patch doesn't.

fixing here.


Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
kernel/trace/trace_workqueue.c | 20 ++++++++++++++------
1 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_workqueue.c b/kernel/trace/trace_workqueue.c
index 46c8dc8..e6c4d00 100644
--- a/kernel/trace/trace_workqueue.c
+++ b/kernel/trace/trace_workqueue.c
@@ -193,12 +193,20 @@ static int workqueue_stat_show(struct seq_file *s, void *p)
struct cpu_workqueue_stats *cws = p;
unsigned long flags;
int cpu = cws->cpu;
- struct task_struct *tsk = find_task_by_vpid(cws->pid);
-
- seq_printf(s, "%3d %6d %6u %s\n", cws->cpu,
- atomic_read(&cws->inserted),
- cws->executed,
- tsk ? tsk->comm : "<...>");
+ struct pid *pid;
+ struct task_struct *tsk;
+
+ pid = find_get_pid(cws->pid);
+ if (pid) {
+ tsk = get_pid_task(pid, PIDTYPE_PID);
+ if (tsk) {
+ seq_printf(s, "%3d %6d %6u %s\n", cws->cpu,
+ atomic_read(&cws->inserted), cws->executed,
+ tsk->comm);
+ put_task_struct(tsk);
+ }
+ put_pid(pid);
+ }

spin_lock_irqsave(&workqueue_cpu_stat(cpu)->lock, flags);
if (&cws->list == workqueue_cpu_stat(cpu)->list.next)
--
1.6.1.2





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