linux-kernel - [PATCH v8 13/14] arch/tile: turn off timer tick for oneshot

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Message-ID: <1445373372-6567-14-git-send-email-cmetcalf@ezchip.com>
Date:	Tue, 20 Oct 2015 16:36:11 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rik van Riel" <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	<linux-kernel@...r.kernel.org>
CC:	Chris Metcalf <cmetcalf@...hip.com>
Subject: [PATCH v8 13/14] arch/tile: turn off timer tick for oneshot_stopped state

When the schedule tick is disabled in tick_nohz_stop_sched_tick(),
we call hrtimer_cancel(), which eventually calls down into
__remove_hrtimer() and thus into hrtimer_force_reprogram().  That
function's call to tick_program_event() detects that we are trying to
set the expiration to KTIME_MAX and calls clockevents_switch_state()
to set the state to ONESHOT_STOPPED, and returns.

However, by default the internal __clockevents_switch_state() code
doesn't have a "set_state_oneshot_stopped" function pointer for the
tile clock_event_device, so that code returns -ENOSYS, and we end up
not setting the state, and more importantly, we don't actually turn
off the tile hardware timer.  As a result, the timer tick we were
waiting for before is still queued, and fires shortly afterwards,
only to discover there was nothing for it to do, at which point
it quiesces.

The fix is to provide that function pointer for tile, and like the
other function pointers, have it just turn off the timer interrupt.
Any call to set a new timer interval will properly re-enable it.

This fix avoids a small performance hiccup for regular applications,
but for TASK_ISOLATION code, it fixes a potentially disastrous
kernel timer interruption that could cause packets to be dropped.

Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
---
 arch/tile/kernel/time.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
index 178989e6d3e3..fbedf380d9d4 100644
--- a/arch/tile/kernel/time.c
+++ b/arch/tile/kernel/time.c
@@ -159,6 +159,7 @@ static DEFINE_PER_CPU(struct clock_event_device, tile_timer) = {
 	.set_next_event = tile_timer_set_next_event,
 	.set_state_shutdown = tile_timer_shutdown,
 	.set_state_oneshot = tile_timer_shutdown,
+	.set_state_oneshot_stopped = tile_timer_shutdown,
 	.tick_resume = tile_timer_shutdown,
 };

-- 
2.1.2

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