Re: [RFC PATCH] mm/hugetlb: Avoid soft lockup in set_max_huge_pages()

[hejianet]

Hi Dave

On 7/26/16 11:58 PM, Dave Hansen wrote:
> On 07/26/2016 08:44 AM, Jia He wrote:
> > This patch is to fix such soft lockup. I thouhgt it is safe to call
> > cond_resched() because alloc_fresh_gigantic_page and alloc_fresh_huge_page
> > are out of spin_lock/unlock section.
> Yikes.  So the call site for both the things you patch is this:
>
> >          while (count > persistent_huge_pages(h)) {
> ...
> >                  spin_unlock(&hugetlb_lock);
> >                  if (hstate_is_gigantic(h))
> >                          ret = alloc_fresh_gigantic_page(h, nodes_allowed);
> >                  else
> >                          ret = alloc_fresh_huge_page(h, nodes_allowed);
> >                  spin_lock(&hugetlb_lock);
> and you choose to patch both of the alloc_*() functions.  Why not just
> fix it at the common call site?  Seems like that
> spin_lock(&hugetlb_lock) could be a cond_resched_lock() which would fix
> both cases.
I agree to move the cond_resched() to a common site in set_max_huge_pages().
But do you mean the spin_lock in this while loop can be replaced by
cond_resched_lock?
IIUC, cond_resched_lock = spin_unlock+cond_resched+spin_lock.
So could you please explain more details about it? Thanks.

B.R.
Justin
> Also, putting that cond_resched() inside the for_each_node*() loop is an
> odd choice.  It seems to indicate that the loops can take a long time,
> which really isn't the case.  The _loop_ isn't long, right?