From: Denys Vlasenko <dvlasenk@redhat.com>
To: "Ingo Molnar" <mingo@kernel.org>,
"Andy Lutomirski" <luto@kernel.org>,
"Dan Williams" <dan.j.williams@intel.com>,
"Alex Deucher" <alexander.deucher@amd.com>,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
"Christian König" <christian.koenig@amd.com>,
"Vinod Koul" <vinod.koul@intel.com>,
"Johannes Berg" <johannes.berg@intel.com>,
"Sara Sharon" <sara.sharon@intel.com>,
"Adrian Hunter" <adrian.hunter@intel.com>,
"Linus Torvalds" <torvalds@linux-foundation.org>
Cc: x86@kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: RFC: Petition Intel/AMD to add POPF_IF insn
Date: Wed, 17 Aug 2016 19:20:37 +0200 [thread overview]
Message-ID: <f2d39812-8e2b-ed32-398f-8bb32bd20c7f@redhat.com> (raw)
Last year, a proposal was floated to avoid costly POPF.
In particular, each spin_unlock_irqrestore() does it,
a rather common operation.
https://lkml.org/lkml/2015/4/21/290
[RFC PATCH] x86/asm/irq: Don't use POPF but STI
* Andy Lutomirski <luto@kernel.org> wrote:
> > Another different approach would be to formally state that
> > pv_irq_ops.save_fl() needs to return all the flags, which would
> > make local_irq_save() safe to use in this circumstance, but that
> > makes a hotpath longer for the sake of a single boot time check.
>
> ...which reminds me:
>
> Why does native_restore_fl restore anything other than IF? A branch
> and sti should be considerably faster than popf.
Ingo agreed:
====
Yes, this has come up in the past, something like the patch below?
Totally untested and not signed off yet: because we'd first have to
make sure (via irq flags debugging) that it's not used in reverse, to
re-disable interrupts:
local_irq_save(flags);
local_irq_enable();
...
local_irq_restore(flags); /* effective local_irq_disable() */
I don't think we have many (any?) such patterns left, but it has to be
checked first. If we have such cases then we'll have to use different
primitives there.
====
Linus replied:
=====
"popf" is fast for the "no changes to IF" case, and is a smaller
instruction anyway.
====
This basically shot down the proposal.
But in my measurements POPF is not fast even in the case where restored
flags are not changes at all:
mov $200*1000*1000, %eax
pushf
pop %rbx
.balign 64
loop: push %rbx
popf
dec %eax
jnz loop
# perf stat -r20 ./popf_1g
4,929,012,093 cycles # 3.412 GHz ( +- 0.02% )
835,721,371 instructions # 0.17 insn per cycle ( +- 0.02% )
1.446185359 seconds time elapsed ( +- 0.46% )
If I replace POPF with a pop into an unused register, I get this:
loop: push %rbx
pop %rcx
dec %eax
jnz loop
209,247,645 cycles # 3.209 GHz ( +- 0.11% )
801,898,016 instructions # 3.83 insn per cycle ( +- 0.00% )
0.066210725 seconds time elapsed ( +- 0.59% )
IOW, POPF takes at least 6 cycles.
Linus does have a point that a "test+branch+STI" may end up not a clear win because of the branch.
But the need to restore IF flag exists, it is necessary not only for Linux, but for any OS
running on x86: they all have some sort of spinlock.
The addition of a POPF instruction variant which looks only at IF bit and changes
only that bit in EFLAGS may be a good idea, for all OSes.
I propose that we ask Intel / AMD to do that.
Maybe by the "ignored prefix" trick which was used when LZCNT insn was introduced
as REPZ-prefixed BSR?
Currently, REPZ POPF (f3 9d) insn does execute. Redefine this opcode as
POPF_IF. Then the same kernel will work on old and new CPUs.
CC'ing some @intel and @amd emails...
next reply other threads:[~2016-08-17 17:21 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-17 17:20 Denys Vlasenko [this message]
2016-08-17 17:30 ` RFC: Petition Intel/AMD to add POPF_IF insn Christian König
2016-08-17 18:34 ` Denys Vlasenko
2016-08-17 17:32 ` Linus Torvalds
2016-08-17 18:41 ` Denys Vlasenko
[not found] ` <CA+55aFwmxwQBkyDjombS4cy1q=a_buhmDjDnaa8rdC8ZDaDYEA@mail.gmail.com>
2016-08-17 19:13 ` Andy Lutomirski
2016-08-17 19:26 ` Denys Vlasenko
2016-08-17 19:32 ` Linus Torvalds
2016-08-17 19:35 ` Denys Vlasenko
2016-08-17 19:54 ` Linus Torvalds
2016-08-17 19:37 ` Linus Torvalds
2016-08-17 21:26 ` Linus Torvalds
2016-08-17 21:35 ` Linus Torvalds
2016-08-17 21:43 ` Andy Lutomirski
2016-08-17 21:48 ` Linus Torvalds
2016-08-18 13:26 ` Denys Vlasenko
2016-08-18 17:24 ` Linus Torvalds
2016-08-18 17:47 ` Denys Vlasenko
2016-08-18 17:49 ` Denys Vlasenko
2016-08-19 10:54 ` Paolo Bonzini
2016-08-31 11:12 ` Denys Vlasenko
2016-08-18 9:21 ` Denys Vlasenko
2016-08-18 12:18 ` Denys Vlasenko
2016-08-18 17:22 ` Paolo Bonzini
2016-08-17 19:29 ` Linus Torvalds
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