From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
"Russell King (Oracle)" <linux@armlinux.org.uk>,
the arch/x86 maintainers <x86@kernel.org>,
Dave Hansen <dave.hansen@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Nicholas Piggin <npiggin@gmail.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
Date: Thu, 17 Jun 2021 16:05:03 +0200 [thread overview]
Message-ID: <YMtWj/y4Hc0CeCux@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <394219d4-36a6-4e7f-a03c-8590551b099a@www.fastmail.com>
On Thu, Jun 17, 2021 at 06:41:41AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 17, 2021, at 4:33 AM, Mark Rutland wrote:
> > Sure, and I agree we should not change cacheflush().
> >
> > The point of membarrier(SYNC_CORE) is that you can move the cost of that
> > ISB out of the fast-path in the executing thread(s) and into the
> > slow-path on the thread which generated the code.
> >
> > So e.g. rather than an executing thread always having to do:
> >
> > LDR <reg>, [<funcptr>]
> > ISB // in case funcptr was just updated
> > BLR <reg>
> >
> > ... you have the thread generating the code use membarrier(SYNC_CORE)
> > prior to plublishing the funcptr, and the fast-path on all the executing
> > threads can be:
> >
> > LDR <reg> [<funcptr>]
> > BLR <reg>
> >
> > ... and thus I think we still want membarrier(SYNC_CORE) so that people
> > can do this, even if there are other means to achieve the same
> > functionality.
>
> I had the impression that sys_cacheflush() did that. Am I wrong?
Yes, sys_cacheflush() only does what it says on the tin (and only
correctly for hardware broadcast -- everything except 11mpcore).
It only invalidates the caches, but not the per CPU derived state like
prefetch buffers and micro-op buffers, and certainly not instructions
already in flight.
So anything OoO needs at the very least a complete pipeline stall
injected, but probably something stronger to make it flush the buffers.
> In any event, I’m even more convinced that no new SYNC_CORE arches
> should be added. We need a new API that just does the right thing.
I really don't understand why you hate the thing so much; SYNC_CORE is a
means of injecting whatever instruction is required to flush all uarch
state related to instructions on all theads (not all CPUs) of a process
as efficient as possible.
The alternative is sending signals to all threads (including the
non-running ones) which is known to scale very poorly indeed, or, as
Mark suggests above, have very expensive instructions unconditinoally in
the instruction stream, which is also undesired.
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-06-17 14:07 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1623813516.git.luto@kernel.org>
2021-06-16 3:21 ` [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE Andy Lutomirski
2021-06-16 9:28 ` Russell King (Oracle)
2021-06-16 10:16 ` Peter Zijlstra
2021-06-16 10:20 ` Peter Zijlstra
2021-06-16 10:34 ` Russell King (Oracle)
2021-06-16 11:10 ` Peter Zijlstra
2021-06-16 13:22 ` Russell King (Oracle)
2021-06-16 15:04 ` Catalin Marinas
2021-06-16 15:23 ` Russell King (Oracle)
2021-06-16 15:45 ` Catalin Marinas
2021-06-16 16:00 ` Catalin Marinas
2021-06-16 16:27 ` Russell King (Oracle)
2021-06-17 8:55 ` Krzysztof Hałasa
2021-06-18 12:54 ` Linus Walleij
2021-06-18 13:19 ` Russell King (Oracle)
2021-06-18 13:36 ` Arnd Bergmann
2021-06-17 10:40 ` Mark Rutland
2021-06-17 11:23 ` Russell King (Oracle)
2021-06-17 11:33 ` Mark Rutland
2021-06-17 13:41 ` Andy Lutomirski
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski
2021-06-17 14:20 ` Mark Rutland
2021-06-17 15:01 ` Peter Zijlstra
2021-06-17 15:13 ` Peter Zijlstra
2021-06-17 14:16 ` Mathieu Desnoyers
2021-06-17 14:05 ` Peter Zijlstra [this message]
2021-06-18 0:07 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
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=YMtWj/y4Hc0CeCux@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=akpm@linux-foundation.org \
--cc=dave.hansen@intel.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux@armlinux.org.uk \
--cc=luto@kernel.org \
--cc=mark.rutland@arm.com \
--cc=mathieu.desnoyers@efficios.com \
--cc=npiggin@gmail.com \
--cc=x86@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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).