Commit 53da1d94 authored by Miklos Szeredi's avatar Miklos Szeredi Committed by Linus Torvalds
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fix ptrace slowness

This patch fixes bug #12208:

  Bug-Entry       :

  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

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: default avatarMiklos Szeredi <>
Signed-off-by: default avatarLinus Torvalds <>
parent b0dcb4a9
......@@ -1575,7 +1575,15 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
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().
} else {
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