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

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Message-ID: <20250226092857.La2UyrrB@linutronix.de>
Date: Wed, 26 Feb 2025 10:28:57 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
	"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>, Paolo Abeni <pabeni@...hat.com>,
	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 2025-02-22 16:13:35 [+0800], Yunsheng Lin 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?

indeed, indeed, what was I thinking…

> 
> > +		} while (u64_stats_fetch_retry(&pcpu->syncp, start));
> >  	}
> >  
> >  	return true;

Sebastian