From: Andi Kleen <andi@firstfloor.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Andy Lutomirski <luto@MIT.EDU>,
Linus Torvalds <torvalds@linux-foundation.org>,
Nick Piggin <npiggin@suse.de>,
"David S. Miller" <davem@davemloft.net>,
Eric Dumazet <eric.dumazet@gmail.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
Andi Kleen <andi@firstfloor.org>,
linux-kernel@vger.kernel.org
Subject: Re: [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers
Date: Thu, 7 Apr 2011 17:23:54 +0200 [thread overview]
Message-ID: <20110407152354.GW21838@one.firstfloor.org> (raw)
In-Reply-To: <20110407082550.GG24879@elte.hu>
> Also, do we *really* have RDTSC SMP-coherency guarantees on multi-socket CPUs
> today? It now works on multi-core, but on bigger NUMA i strongly doubt it. So
> this hack tries to preserve something that we wont be able to offer anyway.
Some larger NUMA systems have explicit TSC consistency in hardware; on those that don't
we disable TSC as a clocksource so this path should be never taken.
> So the much better optimization would be to give up on exact GTOD coherency and
> just make sure the same task does not see time going backwards. If user-space
> wants precise coherency it can use synchronization primitives itsef. By default
> it would get the fast and possibly off by a few cycles thing instead. We'd
> never be seriously jump in time - only small jumps would happen in practice,
> depending on CPU parallelism effects.
That would be a big user visible break in compatibility.
Any small jump can lead to a negative time difference, and negative time differences
are known to break applications.
e.g. typical case is app using this as a event time stamp into a buffer written
from multiple CPUs, and then assuming that the time stamp always goes up.
> If we do that then the optimization would be to RDTSC and not use *any* of the
> barriers, neither the hardware ones nor your tricky software data-dependency
> obfuscation barrier.
The barriers were originally added because a stress test was able to observe
time going backwards without them.
-Andi
--
ak@linux.intel.com -- Speaking for myself only.
next prev parent reply other threads:[~2011-04-07 15:23 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-07 2:03 [RFT/PATCH v2 0/6] Micro-optimize vclock_gettime Andy Lutomirski
2011-04-07 2:03 ` [RFT/PATCH v2 1/6] x86-64: Clean up vdso/kernel shared variables Andy Lutomirski
2011-04-07 8:08 ` Ingo Molnar
2011-04-07 2:03 ` [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers Andy Lutomirski
2011-04-07 8:25 ` Ingo Molnar
2011-04-07 11:44 ` Andrew Lutomirski
2011-04-07 15:23 ` Andi Kleen [this message]
2011-04-07 17:28 ` Ingo Molnar
2011-04-07 16:18 ` Linus Torvalds
2011-04-07 16:42 ` Andi Kleen
2011-04-07 17:20 ` Linus Torvalds
2011-04-07 18:15 ` Andi Kleen
2011-04-07 18:30 ` Linus Torvalds
2011-04-07 21:26 ` Andrew Lutomirski
2011-04-08 17:59 ` Andrew Lutomirski
2011-04-09 11:51 ` Ingo Molnar
2011-04-07 21:43 ` Raghavendra D Prabhu
2011-04-07 22:52 ` Andi Kleen
2011-04-07 2:04 ` [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc Andy Lutomirski
2011-04-07 7:54 ` Ingo Molnar
2011-04-07 11:25 ` Andrew Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 Andy Lutomirski
2011-04-07 7:57 ` Ingo Molnar
2011-04-07 11:27 ` Andrew Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 5/6] x86-64: Move vread_tsc into a new file with sensible options Andy Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 6/6] x86-64: Turn off -pg and turn on -foptimize-sibling-calls for vDSO Andy Lutomirski
2011-04-07 8:03 ` Ingo Molnar
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=20110407152354.GW21838@one.firstfloor.org \
--to=andi@firstfloor.org \
--cc=a.p.zijlstra@chello.nl \
--cc=davem@davemloft.net \
--cc=eric.dumazet@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@MIT.EDU \
--cc=mingo@elte.hu \
--cc=npiggin@suse.de \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
--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).