From: David Laight <David.Laight@ACULAB.COM>
To: 'Nicholas Piggin' <npiggin@gmail.com>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: RE: [RFC PATCH 2/2] powerpc: nop trap instruction after WARN_ONCE fires
Date: Mon, 26 Sep 2022 08:56:00 +0000 [thread overview]
Message-ID: <347d89ae6b7d452b8d8d417857476b7b@AcuMS.aculab.com> (raw)
In-Reply-To: <20220923154143.1115645-2-npiggin@gmail.com>
From: Nicholas Piggin
> Sent: 23 September 2022 16:42
>
> WARN_ONCE and similar are often used in frequently executed code, and
> should not crash the system. The program check interrupt caused by
> WARN_ON_ONCE can be a significant overhead even when nothing is being
> printed. This can cause performance to become unacceptable, having the
> same effective impact to the user as a BUG_ON().
>
> Avoid this overhead by patching the trap with a nop instruction after a
> "once" trap fires. Conditional warnings that return a result must have
> equivalent compare and branch instructions after the trap, so when it is
> nopped the statement will behave the same way. It's possible the asm
> goto should be removed entirely and this comparison just done in C now.
>
> XXX: possibly this should schedule the patching to run in a different
> context than the program check.
I'm pretty sure WARN_ON_ONCE() is valid everywhere printk() is allowed.
In many cases this means you can't call mutex_enter().
So you need a different scheme.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2022-09-26 9:02 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-23 15:41 [RFC PATCH 1/2] powerpc: Don't use extable for WARN Nicholas Piggin
2022-09-23 15:41 ` [RFC PATCH 2/2] powerpc: nop trap instruction after WARN_ONCE fires Nicholas Piggin
2022-09-23 16:47 ` Christophe Leroy
2022-10-04 4:31 ` Nicholas Piggin
2022-10-04 8:14 ` Christophe Leroy
2022-09-26 8:56 ` David Laight [this message]
2022-09-23 15:49 ` [RFC PATCH 1/2] powerpc: Don't use extable for WARN Christophe Leroy
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