Re: [RFC v4 2/2] x86/xen: allow privcmd hypercalls to be preempted

From: Steven Rostedt <rostedt@goodmis.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: X86 ML <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@suse.com>,
	"Luis R. Rodriguez" <mcgrof@do-not-panic.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
	David Vrabel <david.vrabel@citrix.com>,
	Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC v4 2/2] x86/xen: allow privcmd hypercalls to be preempted
Date: Thu, 22 Jan 2015 20:57:17 -0500	[thread overview]
Message-ID: <20150122205717.57615751__15459.8645786338$1421978316$gmane$org@grimm.local.home> (raw)
In-Reply-To: <CALCETrXKeoFKCDYcqeN=HV6MJt_CjwSPQmn_DbBxY6s-76W3Tw@mail.gmail.com>

On Thu, 22 Jan 2015 17:40:27 -0800
Andy Lutomirski <luto@amacapital.net> wrote:

> >
> > +/*
> > + * CONFIG_PREEMPT=n kernels can end up triggering the softlock
> > + * TASK_UNINTERRUPTIBLE hanger check (default 120 seconds)
> > + * when certain multicalls are used [0] on large systems, in
> > + * that case we need a way to voluntarily preempt. This is
> > + * only an issue on CONFIG_PREEMPT=n kernels.
> > + *
> > + * [0] https://bugzilla.novell.com/show_bug.cgi?id=861093
> > + */
> > +void xen_end_upcall(struct pt_regs *regs)
> > +{
> > +       if (xen_is_preemptible_hypercall(regs)) {
> > +               int cpuid = smp_processor_id();
> > +               if (_cond_resched())
> > +                       trace_xen_hypercall_preemption(cpuid);
> 
> If you want to speed this up a bit, I think you could move the
> smp_processor_id() into the TP_fast_assign.  But don't tracepoints
> report the cpu number even without any action?

Yes, but if you scheduled here, the tracepoint could happen on a
different CPU. Thus, cpuid will not equal smp_processor_id().

-- Steve