linux-kernel - Re: [PATCH] Minimal mmu notifiers for kvm

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Date:	Fri, 25 Apr 2008 06:12:43 -0500
From:	Robin Holt <holt@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...ranet.com>,
	Christoph Lameter <clameter@....com>,
	Andrea Arcangeli <andrea@...ranet.com>,
	Robin Holt <holt@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] Minimal mmu notifiers for kvm

This patch would require GRU to maintain its own page tables and hold
reference counts on the pages.  That seems like a complete waste of
memory compared to Andrea's most recent patch.  The invalidate_range_start
and invalidate_range_end pair is needed to eliminate the page reference
counts.  The _start callout sets an internal structure in a state that
prevents GRU from satisfying faults, then executes the GRU instruction
to flush the TLB entry.  The _end callout releases the block on faults.

On Fri, Apr 25, 2008 at 08:13:00AM +1000, Rusty Russell wrote:
> +static DEFINE_SPINLOCK(notifier_lock);
> +
> +/*
> + * Must not hold mmap_sem nor any other VM related lock when calling
> + * this registration function.
> + */
> +int mm_add_notifier_ops(struct mm_struct *mm,
> +			const struct mmu_notifier_ops *mops)
> +{
> +	int err;
> +
> +	spin_lock(&notifier_lock);

This one global lock will get extremely hot when a 4096 MPI rank job
is starting up and every one of them goes to use the GRU at once.  I am
not sure where x86_64 peaks out, but on ia64 going beyond approx 32 cpus
contending for the same lock made starvation a very important issue.

> +	if (mm->mmu_notifier_ops)
> +		err = -EBUSY;

So we can only use one of KVM or GRU or Quadrix or IB or (later) XPMEM
per mm?

> +	else {
> +		mm->mmu_notifier_ops = mops;
> +		err = 0;
> +	}
> +	spin_unlock(&notifier_lock);
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(mm_add_notifier_ops);

Robin
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