Messages in this thread Patch in this message | | | Date | Mon, 23 Mar 2009 16:39:34 GMT | From | Miklos Szeredi <> | Subject | [tip:sched/urgent] sched, ptrace: fix UML and ptrace slowness |
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Commit-ID: 84eef8ca758fa4a68c29f7d752376f6ca6872383 Gitweb: http://git.kernel.org/tip/84eef8ca758fa4a68c29f7d752376f6ca6872383 Author: Miklos Szeredi <miklos@szeredi.hu> AuthorDate: Mon, 23 Mar 2009 16:07:24 +0100 Committer: Ingo Molnar <mingo@elte.hu> CommitDate: Mon, 23 Mar 2009 17:37:38 +0100
sched, ptrace: fix UML and ptrace slowness
This patch fixes bug #12208:
Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12208 Subject : uml is very slow on 2.6.28 host
This turned out to be not a scheduler regression, but an already existing problem in ptrace being triggered by subtle scheduler changes.
The problem is this:
- task A is ptracing task B - task B stops on a trace event - task A is woken up and preempts task B - task A calls ptrace on task B, which does ptrace_check_attach() - this calls wait_task_inactive(), which sees that task B is still on the runq - task A goes to sleep for a jiffy - ...
Since UML does lots of the above sequences, those jiffies quickly add up to make it slow as hell.
This patch solves this by not rescheduling in read_unlock() after ptrace_stop() has woken up the tracer.
Thanks to Oleg Nesterov and Ingo Molnar for the feedback.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz> Cc: oleg@redhat.com Cc: peterz@infradead.org Cc: roland@redhat.com Cc: efault@gmx.de Cc: rjw@sisk.pl Cc: jdike@addtoit.com Cc: user-mode-linux-devel@lists.sourceforge.net Cc: torvalds@linux-foundation.org Cc: akpm@linux-foundation.org Cc: <stable@kernel.org> LKML-Reference: <E1LllkK-0000zF-1h@pomaz-ex.szeredi.hu> Signed-off-by: Ingo Molnar <mingo@elte.hu>
--- kernel/signal.c | 8 ++++++++ 1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c index 2a74fe8..1c88144 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1575,7 +1575,15 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info) read_lock(&tasklist_lock); if (may_ptrace_stop()) { do_notify_parent_cldstop(current, CLD_TRAPPED); + /* + * Don't want to allow preemption here, because + * sys_ptrace() needs this task to be inactive. + * + * XXX: implement read_unlock_no_resched(). + */ + preempt_disable(); read_unlock(&tasklist_lock); + preempt_enable_no_resched(); schedule(); } else { /*
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