kern.smp.maxid error on i386 UP [was: powerd / cpufreq question]
Sergey Kandaurov
pluknet at gmail.com
Wed Apr 20 18:54:59 UTC 2011
On 20 April 2011 21:02, Ian Smith <smithi at nimnet.asn.au> wrote:
> On Wed, 13 Apr 2011, Ian Smith wrote:
> > On Tue, 12 Apr 2011, Daniel Gerzo wrote:
> > > On 11.4.2011 6:08, Ian Smith wrote:
> [..]
> > > > Are those kern.cp_times values as they came, or did you remove trailing
> > > > zeroes? Reason I ask is that on my Thinkpad T23, single-core 1133/733
> > > > MHz, sysctl kern.cp_time shows the usual 5 values, but kern.cp_times has
> > > > the same 5 values for cpu0, but then 5 zeroes for each of cpu1 through
> > > > cpu31, on 8.2-PRE about early January. I need to update the script to
> > > > remove surplus data for non-existing cpus, but wonder if the extra data
> > > > also appeared on your 12 core box?
> > >
> > > I haven't removed anything, it's a pure copy&paste.
> >
> > Thanks. I'll check the single-cpu case again after updating to 8.2-R
>
> Ok, still a problem on at least my i386 single core Thinkpad T23 at
> 8.2-R, since 8.0 I think, certainly evident in a sysctl -a at 8.1-R
>
> FreeBSD t23.smithi.id.au 8.2-RELEASE FreeBSD 8.2-RELEASE #1: Thu Apr 14
> 21:45:47 EST 2011 root at t23.smithi.id.au:/usr/obj/usr/src/sys/GENERIC i386
>
> Verbose dmesg: http://smithi.id.au/t23_dmesg_boot-v.8.2-R.txt
> sysctl -a: http://smithi.id.au/t23_sysctl-a_8.2-R.txt
>
> kern.ccpu: 0
> <cpu count="1" mask="0x1">0</cpu>
> kern.smp.forward_signal_enabled: 1
> kern.smp.topology: 0
> kern.smp.cpus: 1
> kern.smp.disabled: 0
> kern.smp.active: 0
> kern.smp.maxcpus: 32
> kern.smp.maxid: 31 <<<<<<<
> hw.ncpu: 1
>
> kern.cp_times: 38548 1 120437 195677 9660939 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
>
> /usr/src/sys/kern/kern_clock.c:
> return SYSCTL_OUT(req, 0, sizeof(long) * CPUSTATES * (mp_maxid + 1));
>
> Consumers of kern.cp_times like powerd, top, dtrace? and others have to
> loop over 32 cpus, all but one non-existent, and there seem to be many
> places in the kernel doing eg: for (cpu = 0; cpu <= mp_maxid; cpu++) {
> and while CPU_FOREACH / CPU_ABSENT will skip over them, seems wasteful
> at best on machines least likely to have cycles to spare.
>
> eg: powerd parses kern.cp_times to count cpus, wasting cycles adding
> up the 31 'empty' cpus. I haven't explored other userland consumers.
>
> Clearly kern.smp.maxid (ie mp_maxid) should be 0, not 31. On i386,
> non-APIC i386 at least, mp_maxid is not set to (mp_ncpus - 1) as on some
> other archs .. after having being initialised to (MAXCPU - 1) in
> /sys/i386/i386/mp_machdep.c it's never updated for non-smp machines.
>
> I haven't chased all of these rabbits down all of their holes by any
> means, but it seems that making /sys/i386/i386/mp_machdep.c do what it
> says it's gonna do ('with an id of 0') should help. Paste, tabs lost:
>
> int
> cpu_mp_probe(void)
> {
> /*
> * Always record BSP in CPU map so that the mbuf init code works
> * correctly.
> */
> all_cpus = 1;
> if (mp_ncpus == 0) {
> /*
> * No CPUs were found, so this must be a UP system. Setup
> * the variables to represent a system with a single CPU
> * with an id of 0.
> */
> mp_ncpus = 1;
> + mp_maxid = 0;
> return (0);
> }
>
> /* At least one CPU was found. */
> if (mp_ncpus == 1) {
> /*
> * One CPU was found, so this must be a UP system with
> * an I/O APIC.
> */
> + mp_maxid = 0;
> return (0);
> }
>
> /* At least two CPUs were found. */
> return (1);
> }
>
> Note that the second added line above already exists in
> /sys/amd64/amd64/mp_machdep.c, maybe to fix a similar problem, though
> that should only apply to 'a UP system with an I/O APIC'. Maybe better
> could be to fix this in cpu_mp_probe's caller, /sys/kern/subr_smp.c:
>
> static void
> mp_start(void *dummy)
> {
> mtx_init(&smp_ipi_mtx, "smp rendezvous", NULL, MTX_SPIN);
>
> /* Probe for MP hardware. */
> if (smp_disabled != 0 || cpu_mp_probe() == 0) {
> mp_ncpus = 1;
> + mp_maxid = 0;
> all_cpus = PCPU_GET(cpumask);
> return;
> }
>
> cpu_mp_start();
> printf("FreeBSD/SMP: Multiprocessor System Detected: %d CPUs\n",
> mp_ncpus);
> cpu_mp_announce();
> }
>
> I'm probably a long way off base for a solution, but think I've located
> the problem. Thoughts? Is this a known issue? Might any developers
> actually still have a single-cpu i386 system to check this on? :)
>
> Very happy to test any patches etc.
>
Ouch.
Looks like that affects a system with 2 cores as well.
Intel Core2 E7200, 8.2-R i386 SMP:
kern.smp.forward_signal_enabled: 1
kern.smp.topology: 0
kern.smp.cpus: 2
kern.smp.disabled: 0
kern.smp.active: 1
kern.smp.maxcpus: 32
kern.smp.maxid: 31
kern.cp_times: 867360 171 429180 70114 170549535 1385294 306 176659 82618
170270900 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0
Your analysis looks promising.
--
wbr,
pluknet
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