linux-kernel - Re: [PATCH v2 1/3] cacheinfo: Set cache 'id' based on DT data

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Message-ID: <20250711152123.00002153@huawei.com>
Date: Fri, 11 Jul 2025 15:21:23 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Ben Horgan <ben.horgan@....com>
CC: James Morse <james.morse@....com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, "Rafael J . Wysocki" <rafael@...nel.org>,
	<sudeep.holla@....com>, Rob Herring <robh@...nel.org>, Catalin Marinas
	<catalin.marinas@....com>, <WillDeaconwill@...nel.org>
Subject: Re: [PATCH v2 1/3] cacheinfo: Set cache 'id' based on DT data

On Thu, 10 Jul 2025 12:24:01 +0100
Ben Horgan <ben.horgan@....com> wrote:

> Hi James and Jonathan,
> 
> On 7/10/25 12:15, James Morse wrote:
> > Hi Ben, Jonathan,
> > 
> > On 07/07/2025 13:32, Jonathan Cameron wrote:  
> >> On Mon, 7 Jul 2025 11:27:06 +0100
> >> Ben Horgan <ben.horgan@....com> wrote:  
> >>> On 7/4/25 18:38, James Morse wrote:  
> >>>> From: Rob Herring <robh@...nel.org>
> >>>> Use the minimum CPU h/w id of the CPUs associated with the cache for the
> >>>> cache 'id'. This will provide a stable id value for a given system. As
> >>>> we need to check all possible CPUs, we can't use the shared_cpu_map
> >>>> which is just online CPUs. As there's not a cache to CPUs mapping in DT,
> >>>> we have to walk all CPU nodes and then walk cache levels.
> >>>>
> >>>> The cache_id exposed to user-space has historically been 32 bits, and
> >>>> is too late to change. This value is parsed into a u32 by user-space
> >>>> libraries such as libvirt:
> >>>> https://github.com/libvirt/libvirt/blob/master/src/util/virresctrl.c#L1588
> >>>>
> >>>> Give up on assigning cache-id's if a CPU h/w id greater than 32 bits
> >>>> is found.  
> >   
> >>>> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> >>>> index cf0d455209d7..df593da0d5f7 100644
> >>>> --- a/drivers/base/cacheinfo.c
> >>>> +++ b/drivers/base/cacheinfo.c
> >>>> @@ -183,6 +184,42 @@ static bool cache_node_is_unified(struct cacheinfo *this_leaf,
> >>>>    	return of_property_read_bool(np, "cache-unified");
> >>>>    }
> >>>>    
> >>>> +static bool match_cache_node(struct device_node *cpu,
> >>>> +			     const struct device_node *cache_node)
> >>>> +{
> >>>> +	for (struct device_node *cache __free(device_node) = of_find_next_cache_node(cpu);  
> >>> Looks like the creation of this helper function has upset the
> >>> device_node reference counting. This first __free(device_node) will only
> >>> cause of_node_put() to be called in the case of the early return from
> >>> the loop. You've dropped the second __free(device_node) which accounts
> >>> for 'cache' changing on each iteration.  
> > 
> > Heh, I just took this hunk verbatim. Fixing this up with the __free() magic is tricky as
> > the existing patterns all drop the reference to cpu, which we don't want to do here. I
> > think at this point the __free() magic is just making this harder to understand. How about
> > the old fashioned way:
> > 
> > | static bool match_cache_node(struct device_node *cpu,
> > |                              const struct device_node *cache_node)
> > | {
> > |         struct device_node *prev, *cache = of_find_next_cache_node(cpu);
> > |
> > |         while (cache) {
> > |                 if (cache == cache_node) {
> > |                         of_node_put(cache);
> > |                         return true;
> > |                 }
> > |
> > |                 prev = cache;
> > |                 cache = of_find_next_cache_node(cache);
> > |                 of_node_put(prev);
> > |         }
> > |
> > |         return false;
> > | }  
> Ok with me.
Agreed. 

> > 
> >   
> >> Good catch - this behaves differently from many of the of_get_next* type
> >> helpers in that it doesn't drop the reference to the previous iteration
> >> within the call.
> >>
> >> Maybe it should?
> >>
> >> I checked a few of the call sites and some would be simplified if it did
> >> others would need some more complex restructuring but might benefit as
> >> well.  
> > 
> > If it did, we'd end up dropping the reference to cpu on the way in, which
> > of_get_next_cpu_node() in for_each_of_cpu_node() was expecting to do.  
> 
> Yes, I think the blurring of the lines between a cpu node and cache node 
> is at least partially to blame for the confusion here.
Yes.  That is more than a little ugly!

> > 
> > 
> > Thanks,
> > 
> > James  
> 
> Thanks,
> 
> Ben
> 
>