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Date: Mon, 28 Jun 2010 19:31:28 +0900 (JST) From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> To: Rusty Russell <rusty@...tcorp.com.au> Cc: kosaki.motohiro@...fujitsu.com, Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, anton@...ba.org, Mike Travis <travis@....com> Subject: Re: [PATCH 5/5] cpumask: reduce cpumask_size > On Mon, 28 Jun 2010 12:42:23 pm KOSAKI Motohiro wrote: > > > Now we're sure noone is using old cpumask operators, nor *cpumask, we can > > > allocate less bits safely. This reduces the memory usage of off-stack > > > cpumasks when CONFIG_CPUMASK_OFFSTACK=y but we don't have NR_CPUS actual > > > cpus. > > > > I have to say I'm sorry. Probably I broke your assumption. > > If this patch applied, we reintroduce exposing nr_cpu_ids issue and > > break libnuma again. I think following change is necessary too. > > > > Or, Am I missing something? > > I cc'd you because I remembered you being involved in that libnuma issue > and couldn't remember the details. > > Unfortunately, this solution doesn't work: > > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 18faf4d..c14acad 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -4823,7 +4823,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len, > > > > ret = sched_getaffinity(pid, mask); > > if (ret == 0) { > > - size_t retlen = min_t(size_t, len, cpumask_size()); > > + size_t retlen = min_t(size_t, len, > > + BITS_TO_LONGS(NR_CPUS) * sizeof(long)); > > > > Since mask is a cpumask_var_t, only cpumask_size() is allocated. We can't > copy NR_CPUS bits. Ahh, yes. It's purely broken. > But I think it's OK, anyway. libnuma is broken because it gets upset if the > number of cpus it reads from /sys/.../cpumap is more than the cpumask size > returned from sys_sched_getaffinity. > > Currently, getaffinity returns cpumask_size() (ie. based on NR_CPUS), and > the printing routines use nr_cpumask_bits (ie. based on NR_CPUS for > !CPUMASK_OFFSTACK, nr_cpu_ids for CPUMASK_OFFSTACK). > > (libnuma is OK on CONFIG_CPUMASK_OFFSTACK=y because the sysfs output is > *shorter* than expected. I checked the code). > > With this patch, cpumask_size() becomes based on nr_cpumask_bits, so both > getaffinity and sysfs are using the same basis. > > Do you agree? Sure. I agree I missed. Thank you for very kindful explanation! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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