linux-kernel - [PATCH v2] ksoftirqd: Enable IRQs and call cond

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150107014906.GA27996@mail.thefacebook.com>
Date:	Tue, 6 Jan 2015 17:49:06 -0800
From:	Calvin Owens <calvinowens@...com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>, <kernel-team@...com>,
	<calvinowens@...com>
Subject: [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before
 poking RCU

While debugging an issue with excessive softirq usage, I encountered the
following note in commit 3e339b5dae24a706 ("softirq: Use hotplug thread
infrastructure"):

    [ paulmck: Call rcu_note_context_switch() with interrupts enabled. ]

...but despite this note, the patch still calls RCU with IRQs disabled.

This seemingly innocuous change caused a significant regression in softirq
CPU usage on the sending side of a large TCP transfer (~1 GB/s): when
introducing 0.01% packet loss, the softirq usage would jump to around 25%,
spiking as high as 50%. Before the change, the usage would never exceed 5%.

Moving the call to rcu_note_context_switch() after the cond_sched() call,
as it was originally before the hotplug patch, completely eliminated this
problem.

Signed-off-by: Calvin Owens <calvinowens@...com>
---
Changes since v1:
	I mixed up the kernel versions I was patching against, sorry!

 kernel/softirq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 501baa9..9e787d8 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu)
 		 * in the task stack here.
 		 */
 		__do_softirq();
-		rcu_note_context_switch();
 		local_irq_enable();
 		cond_resched();
+
+		preempt_disable();
+		rcu_note_context_switch();
+		preempt_enable();
+
 		return;
 	}
 	local_irq_enable();
-- 
2.1.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/