linux-kernel - [RFC] tsacct: fixup ac_pid field to contain tgid

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Message-Id: <1551789857-19985-1-git-send-email-claudio.fontana@gliwa.com>
Date:   Tue,  5 Mar 2019 13:44:17 +0100
From:   Claudio Fontana <claudio.fontana@...wa.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrey Vagin <avagin@...nvz.org>, linux-kernel@...r.kernel.org,
        Claudio Fontana <claudio.fontana@...il.com>
Subject: [RFC] tsacct: fixup ac_pid field to contain tgid

From: Claudio Fontana <claudio.fontana@...il.com>

commit f3cef7a99469 ("[PATCH] csa: basic accounting over taskstats")
introduced accounting fields over taskstats,

however the ac_pid field has been filled in with the Linux pid
(the tid), instead of the POSIX pid (the tgid) like for acct.c

This change aligns the ac_pid field in tsacct with
the ac_pid field in acct.c

Signed-off-by: Claudio Fontana <claudio.fontana@...il.com>
---
 kernel/tsacct.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Hello all,

this is just to spark some responses regarding the current status
of tsacct field ac_pid, which currently is filled up with pid,
where I would expect a tgid, like for acct.c.

in acct.c:

ac.ac_pid = task_tgid_nr_ns(current, ns);

in tsacct.c:

stats->ac_pid    = task_pid_nr_ns(tsk, pid_ns);

I am trying to have a low overhead way from a C userspace program
to get the tgid given a single pid.

I have tried using the tgid map provided by ftrace, however
parsing and reparsing the text of the whole map is quite expensive.

Reading from /proc/PID/ is of higher overhead, with multiple opens,
reads and close calls necessary.

I have experimented with previous work by Andrey Vagin on
task_diag (the old and the new method), but neither is currently
upstream; one of the comments at the time mentioned that the
functionality should instead be merged into netlink taskstats.

https://criu.org/Task-diag

https://lwn.net/Articles/683371/

I think that for backward compatibility reasons, this particular
change in this RFC cannot be applied as such.

Maybe adding a new field ac_tgid would be preferable?

Maybe the information could be carried inside the NETLINK
taskstat response as an unconditional additional
TASKSTATS_TYPE_TGID field, even when the TASKSTATS_CMD_GET
requests a TASKSTATS_CMD_ATTR_PID?

Thank you and looking forward for your comments,

Claudio

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 370724b..28ecedb 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -59,7 +59,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 		stats->ac_flag |= AXSIG;
 	stats->ac_nice	 = task_nice(tsk);
 	stats->ac_sched	 = tsk->policy;
-	stats->ac_pid	 = task_pid_nr_ns(tsk, pid_ns);
+	stats->ac_pid	 = task_tgid_nr_ns(tsk, pid_ns);
 	rcu_read_lock();
 	tcred = __task_cred(tsk);
 	stats->ac_uid	 = from_kuid_munged(user_ns, tcred->uid);
-- 
2.7.4