Re: 3.0-git15 Atomic scheduling in pidmap_init

Josh Boyer
2011-08-05T01:19:33Z

On Thu, Aug 4, 2011 at 1:31 PM, Josh Boyer <jwboyer@redhat.com> wrote:
> On Thu, Aug 04, 2011 at 09:26:58AM -0700, Paul E. McKenney wrote:
>> > > You really do need to be able to handle set_need_resched() at random
>> > > times, and at first glance it appears to me that the warning could be
>> > > triggered at runtime as well.  If so, the real fix is elsewhere, right?
>> > > Especially given that the patch imposes extra cost at runtime...
>> >
>> > In staring at it for a while it seems to be a combination of being in
>> > atomic context according to the scheduler but things in early boot using
>> > GFP_KERNEL.  At the point we're at in the boot, that is perfectly legal
>> > as it's not being called from an interrupt handler and the mm subsystem
>> > should be all setup, but we're still really early in boot and preempt is
>> > disabled.
>>
>> Isn't preemption disabled at that point in boot?  And isn't GFP_KERNEL
>> illegal within preemption-disabled regions?
>
> Yes, it's disabled.  I'm not sure if it's illegal or not.  pidmap_init
> is called from start_kernel on line 598 of main.c.  local_irq_enable is
> called on line 553, followed immediately by this comment:
>
>        /* Interrupts are enabled now so all GFP allocations are safe. */
>        gfp_allowed_mask = __GFP_BITS_MASK;
>
>        kmem_cache_init_late();
>
> So the comments there lead me to think I have no clue :).  That's mostly
> why I'm asking for feedback here.  I don't have a huge amount of
> experience in what is and isn't allowed in the early bootup path.
>
>> >            As I mentioned before, KMEM_CACHE calls kmalloc with
>> > GFP_KERNEL and I don't think we want to change that.
>> >
>> > Once we're past early boot, I would expect that things running in true
>> > atomic context won't be calling KMEM_CACHE or using GFP_KERNEL.  Or
>> > maybe I hope?
>> >
>> > I understand the desire to avoid another conditional, but I certainly
>> > don't have any other suggestions at the moment.
>>
>> How about doing GFP_ATOMIC on allocations done during that portion
>> of the boot patch for which preemption is disabled?
>
> Well, in the pidmap_init case there are two spots relevant to this.  The
> first is the kzalloc call on line 562 of kernel/pid.c.  I could change
> that to use GFP_IOFS even, and it avoids the backtrace from there. (And
> I did that originally.)
>
> However, the call on line 567 to KMEM_CACHE calls into
> kmem_cache_create.  There is a flags variable, but it's for slab flags,
> and kmem_cache_create calls kmalloc internally with GFP_KERNEL.  I don't
> see kmem_cache_create_atomic or otherwise that would avoid this.  None
> of that code is new either, most if it dating back to 2008.
>
> The same issue exists in some of the next functions called in
> start_kernel, like anon_vma_init, cred_init, fork_init, etc.  They all
> call kmem_cache_create.
>
> I could be missing something obvious, but I don't see a way to avoid
> using GFP_KERNEL without a lot of rip-up in the rest of the init path.

As an aside, I bisected this back to:

e8f7c70f44f sched: Make sleeping inside spinlock detection working in
!CONFIG_PREEMPT

However, that doesn't seem all that helpful. The
CONFIG_DEBUG_SPINLOCK_SLEEP option later got renamed to
DEBUG_ATOMIC_SLEEP, and all it's doing is selecting PREEMPT_COUNT. At
first glance, it seems this commit just allowed an issue that's been
around for a while (benign or otherwise) to finally show up.

(The Fedora kernel configs have CONFIG_PREEMPT_VOLUNTARY set, but not
CONFIG_PREEMPT so PREEMPT_COUNT wasn't getting selected until this
option did so.)

josh