From: Rik van Riel <riel@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>,
williams@redhat.com, Andrew Lutomirski <luto@kernel.org>,
fweisbec@redhat.com, Peter Zijlstra <peterz@infradead.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry
Date: Fri, 01 May 2015 12:22:43 -0400 [thread overview]
Message-ID: <5543A853.6090802@redhat.com> (raw)
In-Reply-To: <CALCETrVZf11EYLhKWOfeQSyzq9eq5KB+btcY19JF+sJvs2zMXA@mail.gmail.com>
On 05/01/2015 12:03 PM, Andy Lutomirski wrote:
> The last time I asked, the impression I got was that we needed two things:
>
> 1. We can't pluck things from the RCU list without knowing whether the
> CPU is in an RCU read-side critical section, and we can't know that
> unless we have regular grade periods or we know that the CPU is idle.
> To make the CPU detectably idle, we need to set a bit somewhere.
More than that. We also need a way for another CPU to identify the
callbacks they could run for us, without confusing them with new
callbacks queued after we transitioned back from USER to KERNEL
context.
> 2. To suppress the timing tick, we need to get some timing for, um,
> the scheduler? I wasn't really sure about this one.
>
> Could we reduce the overhead by making the IN_USER vs IN_KERNEL
> indication be a single bit and, worst case, an rdtsc and maybe a
> subtraction? We could probably get away with banning full nohz on
> non-invariant tsc systems.
I suspect we can.
There is no need to update the vtime every single time
we enter vtime_user_enter and functions like it.
We can keep a buffer, which:
1) keeps values in TSC cycles (or whatever unit local_clock does)
2) is only ever accessed by the current task, so it requires no
locking
3) values can be actually folded into vtime periodically, when
they exceed a certain threshold (1 second ?)
That means the vtime_seqlock is something that we would only take
once a second or so, and the calculations in account_user_time
would only be done on a fairly infrequent basis. That has the
potential to reduce overhead by a lot.
If nobody has any objections to that kind of change, I would be
happy to implement it.
--
All rights reversed
next prev parent reply other threads:[~2015-05-01 16:23 UTC|newest]
Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-30 21:23 [PATCH 0/3] reduce nohz_full syscall overhead by 10% riel
2015-04-30 21:23 ` [PATCH 1/3] reduce indentation in __acct_update_integrals riel
2015-04-30 21:23 ` [PATCH 2/3] remove local_irq_save from __acct_update_integrals riel
2015-04-30 21:23 ` [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry riel
2015-04-30 21:56 ` Andy Lutomirski
2015-05-01 6:40 ` Ingo Molnar
2015-05-01 15:20 ` Rik van Riel
2015-05-01 15:59 ` Ingo Molnar
2015-05-01 16:03 ` Andy Lutomirski
2015-05-01 16:21 ` Ingo Molnar
2015-05-01 16:26 ` Rik van Riel
2015-05-01 16:34 ` Ingo Molnar
2015-05-01 18:05 ` Rik van Riel
2015-05-01 18:40 ` Ingo Molnar
2015-05-01 19:11 ` Rik van Riel
2015-05-01 19:37 ` Andy Lutomirski
2015-05-02 5:27 ` Ingo Molnar
2015-05-02 18:27 ` Rik van Riel
2015-05-03 18:41 ` Andy Lutomirski
2015-05-07 10:35 ` Ingo Molnar
2015-05-04 9:26 ` Paolo Bonzini
2015-05-04 13:30 ` Rik van Riel
2015-05-04 14:06 ` Rik van Riel
2015-05-04 14:19 ` Rik van Riel
2015-05-04 15:59 ` question about RCU dynticks_nesting Rik van Riel
2015-05-04 18:39 ` Paul E. McKenney
2015-05-04 19:39 ` Rik van Riel
2015-05-04 20:02 ` Paul E. McKenney
2015-05-04 20:13 ` Rik van Riel
2015-05-04 20:38 ` Paul E. McKenney
2015-05-04 20:53 ` Rik van Riel
2015-05-05 5:54 ` Paul E. McKenney
2015-05-06 1:49 ` Mike Galbraith
2015-05-06 3:44 ` Mike Galbraith
2015-05-06 6:06 ` Paul E. McKenney
2015-05-06 6:52 ` Mike Galbraith
2015-05-06 7:01 ` Mike Galbraith
2015-05-07 0:59 ` Frederic Weisbecker
2015-05-07 15:44 ` Rik van Riel
2015-05-04 19:00 ` Rik van Riel
2015-05-04 19:39 ` Paul E. McKenney
2015-05-04 19:59 ` Rik van Riel
2015-05-04 20:40 ` Paul E. McKenney
2015-05-05 10:53 ` Peter Zijlstra
2015-05-05 12:34 ` Paul E. McKenney
2015-05-05 13:00 ` Peter Zijlstra
2015-05-05 18:35 ` Paul E. McKenney
2015-05-05 21:09 ` Rik van Riel
2015-05-06 5:41 ` Paul E. McKenney
2015-05-05 10:48 ` Peter Zijlstra
2015-05-05 10:51 ` Peter Zijlstra
2015-05-05 12:30 ` Paul E. McKenney
2015-05-02 4:06 ` [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry Mike Galbraith
2015-05-01 16:37 ` Ingo Molnar
2015-05-01 16:40 ` Rik van Riel
2015-05-01 16:45 ` Ingo Molnar
2015-05-01 16:54 ` Rik van Riel
2015-05-01 17:12 ` Ingo Molnar
2015-05-01 17:22 ` Rik van Riel
2015-05-01 17:59 ` Ingo Molnar
2015-05-01 16:22 ` Rik van Riel [this message]
2015-05-01 16:27 ` Ingo Molnar
2015-05-03 13:23 ` Mike Galbraith
2015-05-03 17:30 ` Rik van Riel
2015-05-03 18:24 ` Andy Lutomirski
2015-05-03 18:52 ` Rik van Riel
2015-05-07 10:48 ` Ingo Molnar
2015-05-07 12:18 ` Frederic Weisbecker
2015-05-07 12:29 ` Ingo Molnar
2015-05-07 15:47 ` Rik van Riel
2015-05-08 7:58 ` Ingo Molnar
2015-05-07 12:22 ` Andy Lutomirski
2015-05-07 12:44 ` Ingo Molnar
2015-05-07 12:49 ` Ingo Molnar
2015-05-08 6:17 ` Paul E. McKenney
2015-05-07 12:52 ` Andy Lutomirski
2015-05-07 15:08 ` Ingo Molnar
2015-05-07 17:47 ` Andy Lutomirski
2015-05-08 6:37 ` Ingo Molnar
2015-05-08 10:59 ` Andy Lutomirski
2015-05-08 11:27 ` Ingo Molnar
2015-05-08 12:56 ` Andy Lutomirski
2015-05-08 13:27 ` Ingo Molnar
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