LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Leonardo Bras <leonardo@linux.ibm.com>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Enrico Weigelt <info@metux.net>,
	Allison Randal <allison@lohutok.net>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash
Date: Fri, 27 Mar 2020 12:51:55 -0300
Message-ID: <56965ad674071181548d5ed4fb7c8fa08061b591.camel@linux.ibm.com> (raw)
In-Reply-To: <af505ef0-e0df-e0aa-bb83-3ed99841f151@c-s.fr>


[-- Attachment #1: Type: text/plain, Size: 1664 bytes --]

Hello Christophe, thanks for the feedback.

I noticed an error in this patch and sent a v2, that can be seen here:
http://patchwork.ozlabs.org/patch/1262468/

Comments inline::

On Fri, 2020-03-27 at 07:50 +0100, Christophe Leroy wrote:
> > @@ -142,6 +144,8 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
> >   		if (likely(__arch_spin_trylock(lock) == 0))
> >   			break;
> >   		do {
> > +			if (unlikely(crash_skip_spinlock))
> > +				return;

Complete function for reference:
static inline void arch_spin_lock(arch_spinlock_t *lock)
{
	while (1) {
		if (likely(__arch_spin_trylock(lock) == 0))
			break;
		do {
			if (unlikely(crash_skip_spinlock))
				return;
			HMT_low();
			if (is_shared_processor())
				splpar_spin_yield(lock);
		} while (unlikely(lock->slock != 0));
		HMT_medium();
	}
}

> You are adding a test that reads a global var in the middle of a so hot 
> path ? That must kill performance. 

I thought it would, in worst case scenario, increase a maximum delay of
an arch_spin_lock() call 1 spin cycle. Here is what I thought:

- If the lock is already free, it would change nothing, 
- Otherwise, the lock will wait.
- Waiting cycle just got bigger.
- Worst case scenario: running one more cycle, given lock->slock can
turn to 0 just after checking.

Could you please point where I failed to see the performance penalty?
(I need to get better at this :) )


> Can we do different ?

Sure, a less intrusive way of doing it would be to free the currently
needed locks before proceeding. I just thought it would be harder to
maintain.

> Christophe

Best regards,
Leonardo

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 22:28 Leonardo Bras
2020-03-26 23:26 ` Leonardo Bras
2020-03-27  6:50 ` Christophe Leroy
2020-03-27 15:51   ` Leonardo Bras [this message]
2020-03-28 10:19     ` Christophe Leroy
2020-03-30 14:33       ` Leonardo Bras
2020-03-30 11:02   ` Peter Zijlstra
2020-03-30 14:12     ` Leonardo Bras

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=56965ad674071181548d5ed4fb7c8fa08061b591.camel@linux.ibm.com \
    --to=leonardo@linux.ibm.com \
    --cc=allison@lohutok.net \
    --cc=benh@kernel.crashing.org \
    --cc=christophe.leroy@c-s.fr \
    --cc=info@metux.net \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mingo@redhat.com \
    --cc=mpe@ellerman.id.au \
    --cc=paulus@samba.org \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lkml.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lkml.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lkml.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lkml.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lkml.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lkml.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lkml.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lkml.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lkml.kernel.org/lkml/8 lkml/git/8.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lkml.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git