Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

Mathieu Desnoyers
2010-01-11T04:35:32Z

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
[...]
> > Even when taking the spinlocks, efficient iteration on active threads is
> > done with for_each_cpu(cpu, mm_cpumask(current->mm)), which depends on
> > the same cpumask, and thus requires the same memory barriers around the
> > updates.
>
> Ouch!!! Good point and good catch!!!
>
> > We could switch to an inefficient iteration on all online CPUs instead,
> > and check read runqueue ->mm with the spinlock held. Is that what you
> > propose ? This will cause reading of large amounts of runqueue
> > information, especially on large systems running few threads. The other
> > way around is to iterate on all the process threads: in this case, small
> > systems running many threads will have to read information about many
> > inactive threads, which is not much better.
>
> I am not all that worried about exactly what we do as long as it is
> pretty obviously correct. We can then improve performance when and as
> the need arises. We might need to use any of the strategies you
> propose, or perhaps even choose among them depending on the number of
> threads in the process, the number of CPUs, and so forth. (I hope not,
> but...)
>
> My guess is that an obviously correct approach would work well for a
> slowpath. If someone later runs into performance problems, we can fix
> them with the added knowledge of what they are trying to do.
>

OK, here is what I propose. Let's choose between two implementations
(v3a and v3b), which implement two "obviously correct" approaches. In
summary:

* baseline (based on 2.6.32.2)
text data bss dec hex filename
76887 8782 2044 87713 156a1 kernel/sched.o

* v3a: ipi to many using mm_cpumask

- adds smp_mb__before_clear_bit()/smp_mb__after_clear_bit() before and
after mm_cpumask stores in context_switch(). They are only executed
when oldmm and mm are different. (it's my turn to hide behind an
appropriately-sized boulder for touching the scheduler). ;) Note that
it's not that bad, as these barriers turn into simple compiler barrier()
on:
avr32, blackfin, cris, frb, h8300, m32r, m68k, mn10300, score, sh,
sparc, x86 and xtensa.
The less lucky architectures gaining two smp_mb() are:
alpha, arm, ia64, mips, parisc, powerpc and s390.
ia64 is gaining only one smp_mb() thanks to its acquire semantic.
- size
text data bss dec hex filename
77239 8782 2044 88065 15801 kernel/sched.o
-> adds 352 bytes of text
- Number of lines (system call source code, w/o comments) : 18

* v3b: iteration on min(num_online_cpus(), nr threads in the process),
taking runqueue spinlocks, allocating a cpumask, ipi to many to the
cpumask. Does not allocate the cpumask if only a single IPI is needed.

- only adds sys_membarrier() and related functions.
- size
text data bss dec hex filename
78047 8782 2044 88873 15b29 kernel/sched.o
-> adds 1160 bytes of text
- Number of lines (system call source code, w/o comments) : 163

I'll reply to this email with the two implementations. Comments are
welcome.

Thanks,

Mathieu

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68