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Date: Tue, 8 Oct 2019 15:11:14 +0000 (UTC)
From: Christopher Lameter <cl@linux.com>
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To: Leonardo Bras <leonardo@linux.ibm.com>
cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, 
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Subject: Re: [PATCH v5 02/11] powerpc/mm: Adds counting method to monitor
 lockless pgtable walks
In-Reply-To: <20191003013325.2614-3-leonardo@linux.ibm.com>
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References: <20191003013325.2614-1-leonardo@linux.ibm.com> <20191003013325.2614-3-leonardo@linux.ibm.com>
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On Wed, 2 Oct 2019, Leonardo Bras wrote:

> +
> +inline unsigned long __begin_lockless_pgtbl_walk(struct mm_struct *mm,
> +						 bool disable_irq)
> +{
> +	unsigned long irq_mask = 0;
> +
> +	if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
> +		atomic_inc(&mm->lockless_pgtbl_walkers);
> +

You are creating contention on a single exclusive cacheline. Doesnt this
defeat the whole purpose of the lockless page table walk? Use mmap_sem or
so should cause the same performance regression?