linux-kernel - Re: [PATCH] rcu/tree: consider time a VM was suspended

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Message-ID: <YKX/H0EwRRLM+cAa@google.com>
Date:   Thu, 20 May 2021 15:18:07 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suleiman Souhlal <suleiman@...gle.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/tree: consider time a VM was suspended

On (21/05/18 16:15), Paul E. McKenney wrote:
> 
> In the shorter term...  PVCLOCK_GUEST_STOPPED is mostly for things like
> guest migration and debugger breakpoints, correct?  Either way, I am
> wondering if rcu_cpu_stall_reset() should take a lighter touch.  Right
> now, it effectively disables all stalls for the current grace period.
> Why not make it restart the stall timeout when the stoppage is detected?

rcu_cpu_stall_reset() is used in many other places, not sure if we can
change its behaviour rcu_cpu_stall_reset().

Maybe it'll be possible to just stop calling it from PV-clock and do
something like this

---

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index eda37df016f0..2d2489eda8e6 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -40,7 +40,7 @@ void pvclock_touch_watchdogs(void)
 {
 	touch_softlockup_watchdog_sync();
 	clocksource_touch_watchdog();
-	rcu_cpu_stall_reset();
+	record_gp_stall_check_time();
 	reset_hung_task_detector();
 }
 
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index cb233c79f0bc..6b3165c7a2c3 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -137,7 +137,7 @@ void rcu_cpu_stall_reset(void)
 // Interaction with RCU grace periods
 
 /* Start of new grace period, so record stall time (and forcing times). */
-static void record_gp_stall_check_time(void)
+void record_gp_stall_check_time(void)
 {
 	unsigned long j = jiffies;
 	unsigned long j1;