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Message-ID: <5543A853.6090802@redhat.com>
Date: Fri, 01 May 2015 12:22:43 -0400
From: Rik van Riel <riel@...hat.com>
To: Andy Lutomirski <luto@...capital.net>,
Ingo Molnar <mingo@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
X86 ML <x86@...nel.org>, williams@...hat.com,
Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
Peter Zijlstra <peterz@...radead.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
& enable from context tracking on syscall entry
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.
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