[PATCH] timers: Stop hrtimer queue traversal after seq_file overflow

[Avi Weiss <thnkslprpt@gmail.com>]

When /proc/timer_list overflows its seq_file buffer, seq_file retries
formatting with a larger buffer. print_active_timers() nevertheless
continues scanning the remaining hrtimers, restarting its quadratic
traversal even though the overflowing attempt will be discarded.

Stop traversing at next_one when the seq_file has overflowed. Preserve
the SysRq console path, which passes a NULL seq_file pointer.

In a (virtual) two-CPU KVM guest with 2,048 timerfds pinned to one CPU,
median reader CPU time for a complete /proc/timer_list read fell from
51.1 ms to 14.6 ms. Repeated tests showed no material slowdown for
non-overflowing reads (13.7us before versus 13.8 after). Short reads,
seek-to-start, and SysRq-Q tests also look fine.

Fixes: 289f480af87e ("[PATCH] Add debugging feature /proc/timer_list")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---
 kernel/time/timer_list.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 0406bf4488e9..84275b6cd2c9 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -64,6 +64,8 @@ static void print_active_timers(struct seq_file *m, struct hrtimer_clock_base *b
 	unsigned long flags;
 
 next_one:
+	if (m && seq_has_overflowed(m))
+		return;
 	i = 0;
 
 	touch_nmi_watchdog();
-- 
2.43.0