Re: [tip:x86/mm] x86/mm/cpa: Avoid wbinvd() for PREEMPT

Ingo Molnar
2017-01-30T16:46:37Z

* tip-bot for John Ogness <tipbot@zytor.com> wrote:

> Commit-ID: 459fbe00693449fade2d1bc802791b081c94edcf
> Gitweb: http://git.kernel.org/tip/459fbe00693449fade2d1bc802791b081c94edcf
> Author: John Ogness <john.ogness@linutronix.de>
> AuthorDate: Mon, 30 Jan 2017 09:41:21 +0100
> Committer: Thomas Gleixner <tglx@linutronix.de>
> CommitDate: Mon, 30 Jan 2017 15:33:52 +0100
>
> x86/mm/cpa: Avoid wbinvd() for PREEMPT
>
> Although wbinvd() is faster than flushing many individual pages, it blocks
> the memory bus for "long" periods of time (>100us), thus directly causing
> unusually large latencies on all CPUs, regardless of any CPU isolation
> features that may be active. This is an unpriviledged operatation as it is
> exposed to user space via the graphics subsystem.

s/unpriviledged
/unprivileged

>
> For 1024 pages, flushing those pages individually can take up to 2200us,
> but the task remains fully preemptible during that time.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Sebastian Siewior <bigeasy@linutronix.de>
> Cc: linux-rt-users <linux-rt-users@vger.kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>
> ---
> arch/x86/mm/pageattr.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 5a287e5..28d4213 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -214,7 +214,20 @@ static void cpa_flush_array(unsigned long *start, int numpages, int cache,
> int in_flags, struct page **pages)
> {
> unsigned int i, level;
> +#ifdef CONFIG_PREEMPT
> + /*
> + * Avoid wbinvd() because it causes latencies on all CPUs,
> + * regardless of any CPU isolation that may be in effect.
> + *
> + * This should be extended for CAT enabled systems independent of
> + * PREEMPT because wbinvd() does not respect the CAT partitions and
> + * this is exposed to unpriviledged users through the graphics
> + * subsystem.

s/unpriviledged
/unprivileged

> +#else
> unsigned long do_wbinvd = cache && numpages >= 1024; /* 4M threshold */
> +#endif

... and while at it:

s/4M/4MB?

Thanks,

Ingo