Date: Mon, 18 May 2015 14:40:56 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Konstantin Belousov <kib@FreeBSD.org>, src-committers@FreeBSD.org, svn-src-all@FreeBSD.org, svn-src-head@FreeBSD.org Subject: Re: svn commit: r282678 - in head: share/man/man4 sys/amd64/acpica sys/amd64/include sys/dev/acpica sys/i386/acpica sys/i386/include sys/x86/include sys/x86/x86 Message-ID: <5559CFC8.3090001@FreeBSD.org> In-Reply-To: <5559CD29.8020106@FreeBSD.org> References: <201505091228.t49CSmVv062442@svn.freebsd.org> <5559CD29.8020106@FreeBSD.org>
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On 18/05/2015 14:29, Andriy Gapon wrote: > On 09/05/2015 15:28, Konstantin Belousov wrote: >> +void >> +acpi_cpu_idle_mwait(uint32_t mwait_hint) >> +{ >> + int *state; >> + >> + state = (int *)PCPU_PTR(monitorbuf); >> + /* >> + * XXXKIB. Software coordination mode should be supported, >> + * but all Intel CPUs provide hardware coordination. >> + */ >> + cpu_monitor(state, 0, 0); >> + cpu_mwait(MWAIT_INTRBREAK, mwait_hint); >> +} >> + > > Kostik, > > it's been a while since I studied this code, so please pardon me if I am asking > something obvious or silly. > > I wonder why this function does not set 'state' before monitor + mwait. > As far as I can see, all other idling functions do that. And cpu_idle_wakeup() > compares the state to STATE_MWAIT before changing it. > > So, I am concerned that if the state happens to be anything other than > STATE_MWAIT when acpi_cpu_idle_mwait() is called, then cpu_idle_wakeup() won't > wake up the idled CPU. It seems that if the state is not STATE_SLEEPING then an > IPI won't be sent either. Actually, that leaves STATE_RUNNING is the only > problematic case, but that's probably the state that the CPU would have before > idling. > After having written the above I realized what I overlooked: acpi_cpu_idle_mwait() is called from the ACPI idle method, so the state must already be set to STATE_SLEEPING. So, looks like the wake-up would always be done by an IPI... Just in case, here's what I had in my old local code: void acpi_cpu_mwait_cx(u_int hints) { int *state; state = (int *)PCPU_PTR(monitorbuf); KASSERT(*state == STATE_SLEEPING, ("cpu_mwait_cx: wrong monitorbuf state")); *state = STATE_MWAIT; cpu_monitor(state, 0, 0); if (*state == STATE_MWAIT) cpu_mwait(MWAIT_INTR_BRK, hints); /* * We should exit on any event that interrupts mwait, * because that event might be a wanted interrupt. */ *state = STATE_RUNNING; } This code also accounted for a time window between the CPU wanting to go idle and it calling mwait. During that window other CPU could want to wake up the first CPU. -- Andriy Gapon
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