Date: Mon, 21 Mar 2016 11:12:57 -0700 From: John Baldwin <jhb@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r297039 - head/sys/x86/x86 Message-ID: <1866602.Bp7VFd5f42@ralph.baldwin.cx> In-Reply-To: <20160319032216.GZ1741@kib.kiev.ua> References: <201603181948.u2IJmndg063765@repo.freebsd.org> <20160319032216.GZ1741@kib.kiev.ua>
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On Saturday, March 19, 2016 05:22:16 AM Konstantin Belousov wrote:
> On Fri, Mar 18, 2016 at 07:48:49PM +0000, John Baldwin wrote:
> >
> > - for (x = 0; x < delay; x += 5) {
> > + for (x = 0; x < delay; x++) {
> > if ((lapic_read_icr_lo() & APIC_DELSTAT_MASK) ==
> > APIC_DELSTAT_IDLE)
> > return (1);
> > - DELAY(5);
> > + DELAY(1);
> > }
> > return (0);
> > }
>
> Ideally we would structure the loop differently. I think it is more
> efficient WRT latency to only block execution by ia32_pause(), and
> compare the the getbinuptime() results to calculate spent time, on each
> loop step.
Yes. I've thought about using the TSC directly to do that, but folks are
worried about the TSC being unstable due to vcpus in a VM migrating
across physical CPUs. DELAY() does seem to DTRT in that case assuming the
hypervisor doesn't advertise an invariant TSC via cpuid. We'd have to
essentially duplicate DELAY() (really delay_tc()) inline.
--
John Baldwin
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