Date: Mon, 25 May 2015 09:02:32 -0600 From: Ian Lepore <ian@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: Andrew Turner <andrew@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r283331 - head/sys/arm/arm Message-ID: <1432566152.1200.34.camel@freebsd.org> In-Reply-To: <1484557.5zjnsBffEc@ralph.baldwin.cx> References: <201505232228.t4NMSxs2032365@svn.freebsd.org> <1484557.5zjnsBffEc@ralph.baldwin.cx>
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On Mon, 2015-05-25 at 07:23 -0400, John Baldwin wrote: > On Saturday, May 23, 2015 10:28:59 PM Andrew Turner wrote: > > Author: andrew > > Date: Sat May 23 22:28:59 2015 > > New Revision: 283331 > > URL: https://svnweb.freebsd.org/changeset/base/283331 > > > > Log: > > Use the wait-for-event instruction to put the core we have just enabled > > to sleep while it waits to start scheduling. The boot core can then use > > the send-event instruction to wake the cores when they should enter the > > scheduler. > > > > MFC after: 1 week > > > > Modified: > > head/sys/arm/arm/mp_machdep.c > > > > Modified: head/sys/arm/arm/mp_machdep.c > > ============================================================================== > > --- head/sys/arm/arm/mp_machdep.c Sat May 23 21:58:41 2015 (r283330) > > +++ head/sys/arm/arm/mp_machdep.c Sat May 23 22:28:59 2015 (r283331) > > @@ -185,8 +185,11 @@ init_secondary(int cpu) > > atomic_add_rel_32(&mp_naps, 1); > > > > /* Spin until the BSP releases the APs */ > > - while (!aps_ready) > > - ; > > + while (!atomic_load_acq_int(&aps_ready)) { > > +#if __ARM_ARCH >= 7 > > + __asm __volatile("wfe"); > > +#endif > > + } > > I don't know that this atomic load acquire is really changing > anything here? Since aps_ready is volatile reading it should > already be "atomic" on each check around the loop. > > > /* Initialize curthread */ > > KASSERT(PCPU_GET(idlethread) != NULL, ("no idle thread")); > > @@ -353,6 +356,10 @@ release_aps(void *dummy __unused) > > arm_unmask_irq(i); > > } > > atomic_store_rel_int(&aps_ready, 1); > > + /* Wake the other threads up */ > > +#if __ARM_ARCH >= 7 > > + armv7_sev(); > > +#endif > > So I'm not at all familiar with these instructions or what they do, > but are the events level triggered? In particular, is there any > sort of race where the sev might arrive in between the check of > aps_ready and the wfe on an AP? (For example, if wfe/sev were > similar to using mwait on x86 for wfe and a memory write for sev, > x86 would require a call to monitor before doing a check of > aps_ready to handle the race like so: > > while (!aps_ready) { > monitor(&aps_ready); > if (!aps_ready) > mwait(); > } > The arm send-event/wait-for-event system includes a 1-bit event latch per core that indicates whether an event arrived since the prior wait. The latch is checked atomically by the hardware as part of going into low-power mode, so you can't get stuck waiting by an event-arrival race, you just need to be prepared to handle spurious wakeups. -- Ian
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