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Message-ID: <20150608100107.GA3135@pathway.suse.cz>
Date: Mon, 8 Jun 2015 12:01:07 +0200
From: Petr Mladek <pmladek@...e.cz>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Richard Weinberger <richard@....at>,
Steven Rostedt <rostedt@...dmis.org>,
David Woodhouse <dwmw2@...radead.org>,
linux-mtd@...ts.infradead.org,
Trond Myklebust <trond.myklebust@...marydata.com>,
Anna Schumaker <anna.schumaker@...app.com>,
linux-nfs@...r.kernel.org, Chris Mason <clm@...com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
Michal Hocko <mhocko@...e.cz>, live-patching@...r.kernel.org,
linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 09/18] kthread: Make it easier to correctly sleep in
iterant kthreads
On Fri 2015-06-05 18:10:21, Peter Zijlstra wrote:
> On Fri, Jun 05, 2015 at 05:01:08PM +0200, Petr Mladek wrote:
> > Many kthreads go into an interruptible sleep when there is nothing
> > to do. They should check if anyone did not requested the kthread
> > to terminate, freeze, or park in the meantime. It is easy to do
> > it a wrong way.
>
> INTERRUPTIBLE is the wrong state to idle in for kthreads, use
> TASK_IDLE.
>
> ---
>
> commit 80ed87c8a9ca0cad7ca66cf3bbdfb17559a66dcf
> Author: Peter Zijlstra <peterz@...radead.org>
> Date: Fri May 8 14:23:45 2015 +0200
>
> sched/wait: Introduce TASK_NOLOAD and TASK_IDLE
>
> Currently people use TASK_INTERRUPTIBLE to idle kthreads and wait for
> 'work' because TASK_UNINTERRUPTIBLE contributes to the loadavg. Having
> all idle kthreads contribute to the loadavg is somewhat silly.
>
> Now mostly this works OK, because kthreads have all their signals
> masked. However there's a few sites where this is causing problems and
> TASK_UNINTERRUPTIBLE should be used, except for that loadavg issue.
>
> This patch adds TASK_NOLOAD which, when combined with
> TASK_UNINTERRUPTIBLE avoids the loadavg accounting.
>
> As most of imagined usage sites are loops where a thread wants to
> idle, waiting for work, a helper TASK_IDLE is introduced.
Just to be sure. Do you suggest to use TASK_IDLE everywhere in
kthreads or only when the uninterruptible sleep is really needed?
IMHO, we should not use TASK_IDLE in freezable kthreads because
it would break freezing. Well, we could freezable_schedule() but only
on locations where it is safe to get freezed. Anyway, we need to
be careful here.
BTW: What is the preferred way of freezing, please? Is it better
to end up in the fridge or is it fine to call freezer_do_not_count();
or set PF_NOFREEZE when it is safe?
The fridge looks more clean to me but in this case we should avoid
uninterruptible sleep as much as possible.
Best Regards,
Petr
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Julian Anastasov <ja@....bg>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: NeilBrown <neilb@...e.de>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index dd07ac03f82a..7de815c6fa78 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -218,9 +218,10 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq);
> #define TASK_WAKEKILL 128
> #define TASK_WAKING 256
> #define TASK_PARKED 512
> -#define TASK_STATE_MAX 1024
> +#define TASK_NOLOAD 1024
> +#define TASK_STATE_MAX 2048
>
> -#define TASK_STATE_TO_CHAR_STR "RSDTtXZxKWP"
> +#define TASK_STATE_TO_CHAR_STR "RSDTtXZxKWPN"
>
> extern char ___assert_task_state[1 - 2*!!(
> sizeof(TASK_STATE_TO_CHAR_STR)-1 != ilog2(TASK_STATE_MAX)+1)];
> @@ -230,6 +231,8 @@ extern char ___assert_task_state[1 - 2*!!(
> #define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED)
> #define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED)
>
> +#define TASK_IDLE (TASK_UNINTERRUPTIBLE | TASK_NOLOAD)
> +
> /* Convenience macros for the sake of wake_up */
> #define TASK_NORMAL (TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE)
> #define TASK_ALL (TASK_NORMAL | __TASK_STOPPED | __TASK_TRACED)
> @@ -245,7 +248,8 @@ extern char ___assert_task_state[1 - 2*!!(
> ((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
> #define task_contributes_to_load(task) \
> ((task->state & TASK_UNINTERRUPTIBLE) != 0 && \
> - (task->flags & PF_FROZEN) == 0)
> + (task->flags & PF_FROZEN) == 0 && \
> + (task->state & TASK_NOLOAD) == 0)
>
> #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
>
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 30fedaf3e56a..d57a575fe31f 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -147,7 +147,8 @@ TRACE_EVENT(sched_switch,
> __print_flags(__entry->prev_state & (TASK_STATE_MAX-1), "|",
> { 1, "S"} , { 2, "D" }, { 4, "T" }, { 8, "t" },
> { 16, "Z" }, { 32, "X" }, { 64, "x" },
> - { 128, "K" }, { 256, "W" }, { 512, "P" }) : "R",
> + { 128, "K" }, { 256, "W" }, { 512, "P" },
> + { 1024, "N" }) : "R",
> __entry->prev_state & TASK_STATE_MAX ? "+" : "",
> __entry->next_comm, __entry->next_pid, __entry->next_prio)
> );
--
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