linux-kernel - Re: [PATCH 1/2] mm/dmapool: replace linked list with xarray

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Date:   Thu, 28 Apr 2022 19:41:03 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com
Subject: Re: [PATCH 1/2] mm/dmapool: replace linked list with xarray

On Thu, Apr 28, 2022 at 10:59:49PM +0100, Matthew Wilcox wrote:
> On Thu, Apr 28, 2022 at 02:27:13PM -0600, kbusch@...nel.org wrote:
> > @@ -316,13 +316,14 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
> >  {
> >  	unsigned long flags;
> >  	struct dma_page *page;
> > +	unsigned long i;
> >  	size_t offset;
> >  	void *retval;
> >  
> >  	might_alloc(mem_flags);
> >  
> >  	spin_lock_irqsave(&pool->lock, flags);
> > -	list_for_each_entry(page, &pool->page_list, page_list) {
> > +	xa_for_each(&pool->pages, i, page) {
> >  		if (page->offset < pool->allocation)
> >  			goto ready;
> >  	}
> 
> A further optimisation you could do is use xarray search marks to
> search for only pages which have free entries.

That's an interesting idea. I didn't consider setting marks since patch 2
replaces this search with essentially a stack pop. If a marked entry can be
returned in a similar time, though, I could drop patch 2. I can't tell from the
xarray code if that operation time is in the same ballpark, though, so I'll
just rerun the the benchmark. :)