Date: Mon, 27 Nov 2017 14:04:59 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r326218 - head/sys/kern Message-ID: <3fc45d5f-22b9-0562-278b-c515e36f48e7@freebsd.org> In-Reply-To: <14322447.103fKFTi3y@ralph.baldwin.cx> References: <201711252341.vAPNf5Qx001464@repo.freebsd.org> <3170692.kvv90QqB0X@ralph.baldwin.cx> <abfdf2b1-bb2f-4eb1-a448-bfee68158577@freebsd.org> <14322447.103fKFTi3y@ralph.baldwin.cx>
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On 11/27/17 11:31, John Baldwin wrote: > On Sunday, November 26, 2017 10:06:56 PM Nathan Whitehorn wrote: >> On 11/26/17 20:50, John Baldwin wrote: >>> On Saturday, November 25, 2017 11:41:05 PM Nathan Whitehorn wrote: >>>> Author: nwhitehorn >>>> Date: Sat Nov 25 23:41:05 2017 >>>> New Revision: 326218 >>>> URL: https://svnweb.freebsd.org/changeset/base/326218 >>>> >>>> Log: >>>> Remove some, but not all, assumptions that the BSP is CPU 0 and that CPUs >>>> are numbered densely from there to n_cpus. >>>> >>>> MFC after: 1 month >>>> >>>> Modified: >>>> head/sys/kern/kern_clock.c >>>> head/sys/kern/kern_clocksource.c >>>> head/sys/kern/kern_shutdown.c >>>> head/sys/kern/kern_timeout.c >>>> head/sys/kern/sched_ule.c >>>> head/sys/kern/subr_pcpu.c >>>> >>>> Modified: head/sys/kern/kern_clock.c >>>> ============================================================================== >>>> --- head/sys/kern/kern_clock.c Sat Nov 25 23:23:24 2017 (r326217) >>>> +++ head/sys/kern/kern_clock.c Sat Nov 25 23:41:05 2017 (r326218) >>>> @@ -573,7 +573,9 @@ hardclock_cnt(int cnt, int usermode) >>>> void >>>> hardclock_sync(int cpu) >>>> { >>>> - int *t = DPCPU_ID_PTR(cpu, pcputicks); >>>> + int *t; >>>> + KASSERT(!CPU_ABSENT(cpu), ("Absent CPU %d", cpu)); >>> Blank line before the KASSERT() perhaps? >>> >>>> + t = DPCPU_ID_PTR(cpu, pcputicks); >>>> >>>> *t = ticks; >>> Probably don't need this blank line though? >> Those are both good ideas. >> >>>> } >>>> >>>> Modified: head/sys/kern/sched_ule.c >>>> ============================================================================== >>>> --- head/sys/kern/sched_ule.c Sat Nov 25 23:23:24 2017 (r326217) >>>> +++ head/sys/kern/sched_ule.c Sat Nov 25 23:41:05 2017 (r326218) >>>> @@ -2444,6 +2451,7 @@ sched_add(struct thread *td, int flags) >>>> * Pick the destination cpu and if it isn't ours transfer to the >>>> * target cpu. >>>> */ >>>> + td_get_sched(td)->ts_cpu = curcpu; /* Pick something valid to start */ >>>> cpu = sched_pickcpu(td, flags); >>> It is not obvious why every sched_add() needs this once you've fixed thread0. >>> Shouldn't new threads just inherit from thread0's already-fixed value? If not, >>> perhaps fix thread0's value sooner? >> That's a fair point. I don't remember the rationale for this now; the >> changes are over a year old from the powernv branch. I do remember >> setting thread0's CPU early not working, but have forgotten why. I will >> try to remember... >> >>>> tdq = sched_setcpu(td, cpu, flags); >>>> tdq_add(tdq, td, flags); >>>> >>>> Modified: head/sys/kern/subr_pcpu.c >>>> ============================================================================== >>>> --- head/sys/kern/subr_pcpu.c Sat Nov 25 23:23:24 2017 (r326217) >>>> +++ head/sys/kern/subr_pcpu.c Sat Nov 25 23:41:05 2017 (r326218) >>>> @@ -279,6 +279,8 @@ pcpu_destroy(struct pcpu *pcpu) >>>> struct pcpu * >>>> pcpu_find(u_int cpuid) >>>> { >>>> + KASSERT(cpuid_to_pcpu[cpuid] != NULL, >>>> + ("Getting uninitialized PCPU %d", cpuid)); >>> This KASSERT seems unnecessary? If the caller uses an invalid one, it will >>> just fault anyway. >> It won't necessarily fault. For example, on PowerPC, NULL is a valid >> address that does not trigger faults. It's unfortunately quite >> complicated to fix this in a general way. Even if it did fault, this >> makes the fault more informative (and has found at least one bug on >> arm64 already). > Given the large number of bugs caused by NULL pointers, having NULL pointers > "work" doesn't seem like a long-term tenable solution. You'd have to go add > explicit KASSERT()'s to every pointer indirection in the kernel which seems > unworkable. Also, adding the KASSERT() here has broken other places that were > depending on the existing behavior of pcpu_find() (see markj@'s commit to dtrace > today). > Unfortunately, it's unfixable on ppc64. Apologies for breaking dtrace! Would you like me to remove the KASSERT() here? I'm happy to do that in a few hours (unless you beat me to it first) -- although I do think that explicitly checking for CPU_ABSENT is a much better behavior in client code than checking the return value of pcpu_find(). -Nathan
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