Messages in this thread Patch in this message | | | From | (Eric W. Biederman) | Date | Wed, 10 Mar 2021 16:40:57 -0600 | Subject | Re: [PATCH v3] do_wait: make PIDTYPE_PID case O(1) instead of O(n) |
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Jim Newsome <jnewsome@torproject.org> writes:
> do_wait is an internal function used to implement waitpid, waitid, > wait4, etc. To handle the general case, it does an O(n) linear scan of > the thread group's children and tracees. > > This patch adds a special-case when waiting on a pid to skip these scans > and instead do an O(1) lookup. This improves performance when waiting on > a pid from a thread group with many children and/or tracees. > > Signed-off-by: James Newsome <jnewsome@torproject.org> > --- > kernel/exit.c | 53 +++++++++++++++++++++++++++++++++++++++++---------- > 1 file changed, 43 insertions(+), 10 deletions(-) > > diff --git a/kernel/exit.c b/kernel/exit.c > index 04029e35e69a..c2438d4ba262 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -1439,9 +1439,34 @@ void __wake_up_parent(struct task_struct *p, struct task_struct *parent) > TASK_INTERRUPTIBLE, p); > } > > +// Optimization for waiting on PIDTYPE_PID. No need to iterate through child > +// and tracee lists to find the target task.
Minor nit: C++ style comments look very out of place in this file which uses old school C /* */ comment delimiters for all of it's block comments. > +static int do_wait_pid(struct wait_opts *wo) > +{ > + struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This is subtle change in behavior.
Today on the task->children list we only place thread group leaders.
Which means that your do_wait_pid wait for thread of someone else's process and that is a change in behavior.
So the code either needs a thread_group_leader filter on target before the ptrace=0 case or we need to use "pid_task(wo->wo_pid, PIDTYPE_TGID)" and "pid_task(wo->wo_pid, PIDTYPE_PID)" for the "ptrace=1" case.
I would like to make thread_group_leaders go away so I would favor two pid_task calls. But either will work right now.
Eric
> + int retval; > + > + if (!target) > + return 0; > + if (current == target->real_parent || > + (!(wo->wo_flags & __WNOTHREAD) && > + same_thread_group(current, target->real_parent))) { > + retval = wait_consider_task(wo, /* ptrace= */ 0, target); > + if (retval) > + return retval; > + } > + if (target->ptrace && (current == target->parent || > + (!(wo->wo_flags & __WNOTHREAD) && > + same_thread_group(current, target->parent)))) { > + retval = wait_consider_task(wo, /* ptrace= */ 1, target); > + if (retval) > + return retval; > + } > + return 0; > +} > + > static long do_wait(struct wait_opts *wo) > { > - struct task_struct *tsk; > int retval; > > trace_sched_process_wait(wo->wo_pid); > @@ -1463,19 +1488,27 @@ static long do_wait(struct wait_opts *wo) > > set_current_state(TASK_INTERRUPTIBLE); > read_lock(&tasklist_lock); > - tsk = current; > - do { > - retval = do_wait_thread(wo, tsk); > - if (retval) > - goto end; > > - retval = ptrace_do_wait(wo, tsk); > + if (wo->wo_type == PIDTYPE_PID) { > + retval = do_wait_pid(wo); > if (retval) > goto end; > + } else { > + struct task_struct *tsk = current; > > - if (wo->wo_flags & __WNOTHREAD) > - break; > - } while_each_thread(current, tsk); > + do { > + retval = do_wait_thread(wo, tsk); > + if (retval) > + goto end; > + > + retval = ptrace_do_wait(wo, tsk); > + if (retval) > + goto end; > + > + if (wo->wo_flags & __WNOTHREAD) > + break; > + } while_each_thread(current, tsk); > + } > read_unlock(&tasklist_lock); > > notask:
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