linux-kernel - Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

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Date:   Mon, 4 May 2020 08:31:47 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

On Mon, May 04, 2020 at 04:21:53PM +0200, Uladzislau Rezki wrote:
> > > > 
> > > > If we are not doing single-pointer allocation, then that would also eliminate
> > > > entering the low-level page allocator for single-pointer allocations.
> > > > 
> > > > Or did you mean entry into the allocator for the full-page allocations
> > > > related to the pointer array for PREEMPT_RT? Even if we skip entry into the
> > > > allocator for those, we will still have additional caching which further
> > > > reduces chances of getting a full page. In the event of such failure, we can
> > > > simply queue the rcu_head.
> > > > 
> > > > Thoughts?
> > > 
> > > I was just trying to guess why you kept the single-pointer allocation.
> > > It looks like I guessed wrong.  ;-)
> > > 
> > > If, as you say above, you make it go straight to synchronize_rcu()
> > > upon full-page allocation failure, that would be good!
> > 
> > Paul, sounds good. Vlad, are you also Ok with that?
> > 
> OK, let's drop it and keep it simple :)
> 
> BTW, for PREEMPT_RT we still can do a page allocation for single
> argument of kvfree_rcu(). In case of double we just revert everything
> to the rcu_head if no cache.
> 
> For single argument we can drop the lock before the entry to the page
> allocator. Because it follows might_sleep() anotation we avoid of having
> a situation when spinlock(rt mutex) is taken from any atomic context.
> 
> Since the lock is dropped the current context can be interrupted by
> an IRQ which in its turn can also call kvfree_rcu() on current CPU.
> In that case it must be double argument(single is not allowed) kvfree_rcu()
> call. For PREEMPT_RT if no cache everything is reverted to rcu_head usage,
> i.e. the entry to page allocator is bypassed.
> 
> It can be addressed as a separate patch and send out later on if we
> are on the same page.
> 
> Paul, Joel what are your opinions?

I strongly prefer that it be removed from the series.  I do understand
that this is a bit more hassle right now, but this does help avoid
confusion in the future, plus perhaps also avoiding issues with future
bisections.

							Thanx, Paul