netdev - Re: [PATCH net-next v6 2/5] net: napi: add CPU affinity to napi

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Message-ID: <34b3a581-a64b-4518-8aab-1f7640c74d56@intel.com>
Date: Mon, 3 Feb 2025 14:32:52 -0700
From: Ahmed Zaki <ahmed.zaki@...el.com>
To: Joe Damato <jdamato@...tly.com>, <netdev@...r.kernel.org>,
	<intel-wired-lan@...ts.osuosl.org>, <andrew+netdev@...n.ch>,
	<edumazet@...gle.com>, <kuba@...nel.org>, <horms@...nel.org>,
	<pabeni@...hat.com>, <davem@...emloft.net>, <michael.chan@...adcom.com>,
	<tariqt@...dia.com>, <anthony.l.nguyen@...el.com>,
	<przemyslaw.kitszel@...el.com>, <shayd@...dia.com>,
	<akpm@...ux-foundation.org>, <shayagr@...zon.com>,
	<kalesh-anakkur.purayil@...adcom.com>
Subject: Re: [PATCH net-next v6 2/5] net: napi: add CPU affinity to
 napi_config



On 2025-01-23 1:18 p.m., Joe Damato wrote:

Sorry for the late reply. I was pulled into other stuff and next was 
closed anyway.

> On Fri, Jan 17, 2025 at 05:33:32PM -0700, Ahmed Zaki wrote:
>> A common task for most drivers is to remember the user-set CPU affinity
>> to its IRQs. On each netdev reset, the driver should re-assign the
>> user's settings to the IRQs.
>>
>> Add CPU affinity mask to napi_config. To delegate the CPU affinity
>> management to the core, drivers must:
>>   1 - set the new netdev flag "irq_affinity_auto":
>>                                         netif_enable_irq_affinity(netdev)
>>   2 - create the napi with persistent config:
>>                                         netif_napi_add_config()
>>   3 - bind an IRQ to the napi instance: netif_napi_set_irq()
>>
>> the core will then make sure to use re-assign affinity to the napi's
>> IRQ.
>>
>> The default IRQ mask is set to one cpu starting from the closest NUMA.
> 
> Maybe the above is helpful to add to
> Documentation/networking/napi.rst ?
> 
>> Signed-off-by: Ahmed Zaki <ahmed.zaki@...el.com>
>> ---
>>   include/linux/netdevice.h | 14 ++++++++++-
>>   net/core/dev.c            | 51 +++++++++++++++++++++++++++++----------
>>   2 files changed, 51 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 98259f19c627..d576e5c91c43 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -351,6 +351,7 @@ struct napi_config {
>>   	u64 gro_flush_timeout;
>>   	u64 irq_suspend_timeout;
>>   	u32 defer_hard_irqs;
>> +	cpumask_t affinity_mask;
>>   	unsigned int napi_id;
>>   };
>>   
>> @@ -393,8 +394,8 @@ struct napi_struct {
>>   	struct list_head	dev_list;
>>   	struct hlist_node	napi_hash_node;
>>   	int			irq;
>> -#ifdef CONFIG_RFS_ACCEL
>>   	struct irq_affinity_notify notify;
>> +#ifdef CONFIG_RFS_ACCEL
>>   	int			napi_rmap_idx;
>>   #endif
>>   	int			index;
>> @@ -1991,6 +1992,11 @@ enum netdev_reg_state {
>>    *
>>    *	@threaded:	napi threaded mode is enabled
>>    *
>> + *	@irq_affinity_auto: driver wants the core to manage the IRQ affinity.
>> + *			    Set by netif_enable_irq_affinity(), then driver must
>> + *			    create persistent napi by netif_napi_add_config()
>> + *			    and finally bind napi to IRQ (netif_napi_set_irq).
>> + *
>>    *	@rx_cpu_rmap_auto: driver wants the core to manage the ARFS rmap.
>>    *	                   Set by calling netif_enable_cpu_rmap().
>>    *
>> @@ -2401,6 +2407,7 @@ struct net_device {
>>   	struct lock_class_key	*qdisc_tx_busylock;
>>   	bool			proto_down;
>>   	bool			threaded;
>> +	bool			irq_affinity_auto;
>>   	bool			rx_cpu_rmap_auto;
>>   
>>   	/* priv_flags_slow, ungrouped to save space */
>> @@ -2653,6 +2660,11 @@ static inline void netdev_set_ml_priv(struct net_device *dev,
>>   	dev->ml_priv_type = type;
>>   }
>>   
>> +static inline void netif_enable_irq_affinity(struct net_device *dev)
>> +{
>> +	dev->irq_affinity_auto = true;
>> +}
> 
> I'll have to look at the patches which use the above function, but
> the first thing that came to mind when I saw this was does the above
> need a WRITE_ONCE ?
> 

Why?

> The reads below seem to be protected by a lock; I haven't yet looked
> at the other patches so maybe the write is also protected by
> netdev->lock ?

All functions are protected by netdev_lock/unlock. Except notifier 
callback (netif_napi_irq_notify) which is protected by the fact that all 
napis are disabled (so IRQ notifier is removed by cancel_work_sync) 
before the napis are deleted.

Other nits are OK IMO and will be fixed in the next version.

Thanks.