linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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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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