netdev - Re: [PATCH net-next 1/2] page_pool: Convert page_pool_recycle_stats to u64_stats

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Message-ID: <50019760-440b-4b0c-816f-d262f747a555@redhat.com>
Date: Tue, 25 Feb 2025 12:27:15 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Yunsheng Lin <linyunsheng@...wei.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-rdma@...r.kernel.org, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Andrew Lunn <andrew+netdev@...n.ch>, Eric Dumazet <edumazet@...gle.com>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>,
 Leon Romanovsky <leon@...nel.org>, Saeed Mahameed <saeedm@...dia.com>,
 Simon Horman <horms@...nel.org>, Tariq Toukan <tariqt@...dia.com>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next 1/2] page_pool: Convert page_pool_recycle_stats
 to u64_stats_t.

On 2/22/25 9:13 AM, Yunsheng Lin wrote:
> On 2025/2/21 19:52, Sebastian Andrzej Siewior wrote:
>> @@ -99,11 +106,19 @@ bool page_pool_get_stats(const struct page_pool *pool,
>>  		const struct page_pool_recycle_stats *pcpu =
>>  			per_cpu_ptr(pool->recycle_stats, cpu);
>>  
>> -		stats->recycle_stats.cached += pcpu->cached;
>> -		stats->recycle_stats.cache_full += pcpu->cache_full;
>> -		stats->recycle_stats.ring += pcpu->ring;
>> -		stats->recycle_stats.ring_full += pcpu->ring_full;
>> -		stats->recycle_stats.released_refcnt += pcpu->released_refcnt;
>> +		do {
>> +			start = u64_stats_fetch_begin(&pcpu->syncp);
>> +			u64_stats_add(&stats->recycle_stats.cached,
>> +				      u64_stats_read(&pcpu->cached));
>> +			u64_stats_add(&stats->recycle_stats.cache_full,
>> +				      u64_stats_read(&pcpu->cache_full));
>> +			u64_stats_add(&stats->recycle_stats.ring,
>> +				      u64_stats_read(&pcpu->ring));
>> +			u64_stats_add(&stats->recycle_stats.ring_full,
>> +				      u64_stats_read(&pcpu->ring_full));
>> +			u64_stats_add(&stats->recycle_stats.released_refcnt,
>> +				      u64_stats_read(&pcpu->released_refcnt));
> 
> It seems the above u64_stats_add() may be called more than one time
> if the below u64_stats_fetch_retry() returns true, which might mean
> the stats is added more than it is needed.
> 
> It seems more correct to me that pool->alloc_stats is read into a
> local varible in the while loop and then do the addition outside
> the while loop?

I also think the above code is incorrect, and local variables to store
the read stats are needed.

/P