linux-kernel - Re: [PATCH RFC v2 07/10] slab: determine barn status racily outside of lock

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Message-ID: <f495fc2a-00e1-4c76-a562-1e7f42a5fbea@suse.cz>
Date: Wed, 12 Mar 2025 19:23:24 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Christoph Lameter
 <cl@...ux.com>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Uladzislau Rezki <urezki@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
 maple-tree@...ts.infradead.org
Subject: Re: [PATCH RFC v2 07/10] slab: determine barn status racily outside
 of lock

On 2/25/25 09:54, Harry Yoo wrote:
> On Fri, Feb 14, 2025 at 05:27:43PM +0100, Vlastimil Babka wrote:
>> The possibility of many barn operations is determined by the current
>> number of full or empty sheaves. Taking the barn->lock just to find out
>> that e.g. there are no empty sheaves results in unnecessary overhead and
>> lock contention. Thus perform these checks outside of the lock with a
>> data_race() annotated variable read and fail quickly without taking the
>> lock.
>> 
>> Checks for sheaf availability that racily succeed have to be obviously
>> repeated under the lock for correctness, but we can skip repeating
>> checks if there are too many sheaves on the given list as the limits
>> don't need to be strict.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> 
> in kmem_cache_return_sheaf:
>> if (!pcs->spare) {                                                      
>> 	pcs->spare = sheaf;                                             
>>	sheaf = NULL;                                                   
>> } else if (pcs->barn->nr_full >= MAX_FULL_SHEAVES) {                    
>>	/* racy check */                                                
>>	barn = pcs->barn;                                               
>>	keep = true;                                                    
>> }  
> 
> By the way this code also needs data_race()?

Right, will add, thanks.