From: David Woodhouse <dwmw2@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
"Hillenbrand, Marius" <mhillenb@amazon.com>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested
Date: Wed, 11 Jul 2018 11:57:43 +0100 [thread overview]
Message-ID: <1531306663.8759.39.camel@infradead.org> (raw)
In-Reply-To: <20180709220823.GA18045@linux.vnet.ibm.com>
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On Mon, 2018-07-09 at 15:08 -0700, Paul E. McKenney wrote:
>
> And the earlier patch was against my -rcu tree, which won't be all that
> helpful for v4.15. Please see below for a lightly tested backport to v4.15.
>
> It should apply to all the releases of interest. If other backports
> are needed, please remind me of my woodhouse.v4.15.2018.07.09a tag.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 6361b81827a8f93f582124da385258fc04a38a7f
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date: Mon Jul 9 13:47:30 2018 -0700
>
> rcu: Make need_resched() respond to urgent RCU-QS needs
>
> The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
> need for an RCU quiescent state from the force-quiescent-state processing
> within the grace-period kthread to context switches and to cond_resched().
> Unfortunately, such urgent needs are not communicated to need_resched(),
> which is sometimes used to decide when to invoke cond_resched(), for
> but one example, within the KVM vcpu_run() function. As of v4.15, this
> can result in synchronize_sched() being delayed by up to ten seconds,
> which can be problematic, to say nothing of annoying.
>
> This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
> rcu_check_callbacks(), which is invoked from the scheduling-clock
> interrupt handler. If the current task is not an idle task and is
> not executing in usermode, a context switch is forced, and either way,
> the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current
> task is an idle task, then RCU's dyntick-idle code will detect the
> quiescent state, so no further action is required. Similarly, if the
> task is executing in usermode, other code in rcu_check_callbacks() and
> its called functions will report the corresponding quiescent state.
>
> Reported-by: David Woodhouse <dwmw2@infradead.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> [ paulmck: Backported to v4.15. Probably applies elsewhere. ]
Hm, this doesn't appear to work. I'm still seeing latencies of 4-5
seconds in my testing. In fact, even our old workaround of adding
rcu_all_qs() into vcpu_enter_guest() didn't properly fix it AFAICT.
I'm just creating a VM with lots of CPUs, then attaching new devices to
it to cause the VMM to open more file descriptors, until it hits a
power of two and invokes expand_fdtable().
expand_fdtable (512) sync took 10472394964 cycles (3500000 µs).
expand_fdtable (512) sync took 15298908072 cycles (5100000 µs).
--- a/fs/file.c
+++ b/fs/file.c
@@ -162,8 +162,16 @@ static int expand_fdtable(struct files_struct *files, unsigned int nr)
/* make sure all __fd_install() have seen resize_in_progress
* or have finished their rcu_read_lock_sched() section.
*/
- if (atomic_read(&files->count) > 1)
+ if (atomic_read(&files->count) > 1) {
+ unsigned long sync_start, sync_end;
+ unsigned long j_start, j_end;
+ j_start = jiffies;
+ sync_start = get_cycles();
synchronize_sched();
+ sync_end = get_cycles();
+ j_end = jiffies;
+ printk("expand_fdtable (%d) sync took %ld cycles (%ld µs).\n", nr, sync_end - sync_start, jiffies_to_usecs(j_end - j_start));
+ }
spin_lock(&files->file_lock);
if (!new_fdt)
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next prev parent reply other threads:[~2018-07-11 10:57 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11 ` Paul E. McKenney
2018-07-06 17:14 ` David Woodhouse
2018-07-06 21:12 ` Paul E. McKenney
2018-07-09 8:58 ` Peter Zijlstra
2018-07-09 8:53 ` Peter Zijlstra
2018-07-09 9:18 ` David Woodhouse
2018-07-09 10:44 ` Peter Zijlstra
2018-07-09 10:56 ` David Woodhouse
2018-07-09 11:06 ` Peter Zijlstra
2018-07-09 11:12 ` David Woodhouse
2018-07-09 11:31 ` Peter Zijlstra
2018-07-09 12:34 ` Paul E. McKenney
2018-07-09 12:47 ` David Woodhouse
2018-07-09 14:30 ` Paul E. McKenney
2018-07-09 12:55 ` Peter Zijlstra
2018-07-09 12:57 ` David Woodhouse
2018-07-09 13:02 ` Peter Zijlstra
2018-07-09 14:29 ` Paul E. McKenney
2018-07-09 14:43 ` Peter Zijlstra
2018-07-09 14:54 ` Paul E. McKenney
2018-07-09 15:26 ` Peter Zijlstra
2018-07-09 16:34 ` Paul E. McKenney
2018-07-09 16:44 ` Paul E. McKenney
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney
2018-07-09 20:35 ` David Woodhouse
2018-07-09 20:42 ` Paul E. McKenney
2018-07-09 20:45 ` David Woodhouse
2018-07-09 21:05 ` Paul E. McKenney
2018-07-09 22:08 ` Paul E. McKenney
2018-07-11 10:57 ` David Woodhouse [this message]
2018-07-11 12:51 ` Paul E. McKenney
2018-07-11 12:58 ` David Woodhouse
2018-07-11 14:25 ` Paul E. McKenney
2018-07-11 14:23 ` David Woodhouse
2018-07-11 14:43 ` Paul E. McKenney
2018-07-11 16:49 ` Paul E. McKenney
2018-07-11 17:03 ` David Woodhouse
2018-07-11 17:48 ` Paul E. McKenney
2018-07-11 18:01 ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20 ` Paul E. McKenney
2018-07-11 18:36 ` Paul E. McKenney
2018-07-11 18:39 ` Christian Borntraeger
2018-07-11 20:27 ` Paul E. McKenney
2018-07-11 20:54 ` David Woodhouse
2018-07-11 21:09 ` Paul E. McKenney
2018-07-11 21:11 ` Christian Borntraeger
2018-07-11 21:32 ` Paul E. McKenney
2018-07-11 21:39 ` Christian Borntraeger
2018-07-11 23:47 ` Paul E. McKenney
2018-07-12 8:31 ` David Woodhouse
2018-07-12 11:00 ` Christian Borntraeger
2018-07-12 11:10 ` David Woodhouse
2018-07-12 11:58 ` Christian Borntraeger
2018-07-12 12:04 ` Christian Borntraeger
2018-07-11 23:37 ` Paul E. McKenney
2018-07-12 2:15 ` Paul E. McKenney
2018-07-12 6:21 ` Christian Borntraeger
2018-07-12 9:52 ` David Woodhouse
2018-07-11 18:31 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17 ` Paul E. McKenney
2018-07-11 20:19 ` David Woodhouse
2018-07-11 21:08 ` Paul E. McKenney
2018-07-12 12:00 ` David Woodhouse
2018-07-12 12:53 ` Paul E. McKenney
2018-07-12 16:17 ` Paul E. McKenney
2018-07-16 15:40 ` Paul E. McKenney
2018-07-17 8:19 ` David Woodhouse
2018-07-17 12:56 ` Paul E. McKenney
2018-07-18 15:36 ` Paul E. McKenney
2018-07-18 16:01 ` David Woodhouse
2018-07-18 16:37 ` Paul E. McKenney
2018-07-18 19:41 ` David Woodhouse
2018-07-18 20:17 ` Paul E. McKenney
2018-07-19 0:26 ` Frederic Weisbecker
2018-07-19 6:45 ` Christian Borntraeger
2018-07-19 7:20 ` David Woodhouse
2018-07-19 10:23 ` Christian Borntraeger
2018-07-19 12:55 ` Paul E. McKenney
2018-07-19 13:14 ` Frederic Weisbecker
2018-07-19 13:36 ` David Woodhouse
2018-07-19 17:09 ` Paul E. McKenney
2018-07-23 8:08 ` David Woodhouse
2018-07-23 12:22 ` Paul E. McKenney
2018-07-19 0:32 ` Frederic Weisbecker
2018-07-19 3:11 ` Paul E. McKenney
2018-07-19 6:16 ` David Woodhouse
2018-07-19 13:17 ` Frederic Weisbecker
2018-07-19 13:15 ` Frederic Weisbecker
2018-07-10 9:24 ` Peter Zijlstra
2018-07-10 16:26 ` Paul E. McKenney
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