linux-kernel - Re: [PATCH 4/4] MSR: msr Batch processing feature

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Message-ID: <CALCETrWOotGkYge_j_TcDUF7S4pBD73WQChBBwRR4phYBdHJfg@mail.gmail.com>
Date:	Thu, 25 Feb 2016 18:48:35 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Marty McFadden <mcfadden8@...l.gov>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
	Brian Gerst <brgerst@...il.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	George Spelvin <linux@...izon.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Pavel Machek <pavel@....cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	Chen Yu <yu.c.chen@...el.com>
Subject: Re: [PATCH 4/4] MSR: msr Batch processing feature

On Thu, Feb 25, 2016 at 4:02 PM, Marty McFadden <mcfadden8@...l.gov> wrote:
> Provides a new ioctl interface through /dev/cpu/msr_batch.
>
> This implementation will cause an Inter Processor Interrupt to be sent
> to each destination processor and will wait until all processors have
> finished processing their respective batch of MSR operations before
> returning.

This comes up periodicially, and it needs to be benchmarked against
the sane way to do it in userspace:

For each CPU: sched_setaffinity to bind to that CPU, and then a bunch
of conventional MSR accesses.  Optionally do all the CPUs at once
using lots of threads.

Please do this benchmark on a kernel that is *not* using nohz_full.
The nohz_full implementaion in current kernels has abysmal syscall
performance, but that's improving rapidly.

--Andy