Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH] x86: don't ever patch back to UP if we unplug cpus. | Date | Mon, 30 Jul 2012 10:45:04 +0930 |
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On Fri, 27 Jul 2012 13:28:29 -0700, Suresh Siddha <suresh.b.siddha@intel.com> wrote: > On Fri, 2012-07-27 at 17:08 +0930, Rusty Russell wrote: > > Paul McKenney points out: > > > > mean offline overhead is 6251/48=130.2 milliseconds. > > > > If I remove the alternatives_smp_switch() from the offline > > path [...] the mean offline overhead is 550/42=13.1 milliseconds > > > > Basically, we're never going to get those 120ms back, and the code is > > pretty messy. > > I am ok with this proposal. If I follow correctly, you are still > allowing the patching to UP happen on boot if there is only one online > cpu currently but a possiblity of multiple cpu's that can come online > later. Perhaps make this little more explicit in the changelog.
That's right. How's this:
Subject: x86: don't ever patch back to UP if we unplug cpus.
We still patch SMP instructions to UP variants if we boot with a single CPU, but not at any other time. In particular, not if we unplug CPUs to return to a single cpu.
Paul McKenney points out:
mean offline overhead is 6251/48=130.2 milliseconds.
If I remove the alternatives_smp_switch() from the offline path [...] the mean offline overhead is 550/42=13.1 milliseconds
Basically, we're never going to get those 120ms back, and the code is pretty messy.
We get rid of: 1) The "smp-alt-once" boot option. It's actually "smp-alt-boot", the documentation is wrong. It's now the default. 2) The skip_smp_alternatives flag used by suspend. 3) arch_disable_nonboot_cpus_begin() and arch_disable_nonboot_cpus_end() which were only used to set this one flag.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> Also, > > > + /* Patch to UP if other cpus not imminent. */ > > + if (noreplace_smp || num_present_cpus() == 1 || setup_max_cpus <= 1) { > > shouldn't this be > > if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1))
Yes, good point.
> also, to be consistent with other checks, may be just use > "num_possible_cpus() == 1" check instead of "setup_max_cpus <= 1".
I think that should work. Will test variations...
Thanks, Rusty.
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