Re: [PATCH 2/2] drivers: base: cacheinfo: Update cpu_map_populated during CPU Hotplug

[Yicong Yang]

Hi Prateek,

On 2023/5/8 16:41, K Prateek Nayak wrote:
> Until commit 5c2712387d48 ("cacheinfo: Fix LLC is not exported through
> sysfs"), cacheinfo called populate_cache_leaves() for CPU coming online
> which let the arch specific functions handle (at least on x86)
> populating the shared_cpu_map. However, with the changes in the
> aforementioned commit, populate_cache_leaves() is not called when a CPU
> comes online as a result of hotplug since last_level_cache_is_valid()
> returns true as the cacheinfo data is not discarded. The CPU coming

Yes in free_cache_attributes() we only update the shared_cpu_map but make
other attributes remained. From my feelings we should do all the work
opposite to detect_cache_attributes(), including free the memory allocated.

> online is not present in shared_cpu_map, however, it will not be added
> since the cpu_cacheinfo->cpu_map_populated flag is set (it is set in
> populate_cache_leaves() when cacheinfo is first populated for x86)
> 
> This can lead to inconsistencies in the shared_cpu_map when an offlined
> CPU comes online again. Example below depicts the inconsistency in the
> shared_cpu_list in cacheinfo when CPU8 is offlined and onlined again on
> a 3rd Generation EPYC processor:
> 
>   # for i in /sys/devices/system/cpu/cpu8/cache/index*/shared_cpu_list; do echo -n "$i: "; cat $i; done
>     /sys/devices/system/cpu/cpu8/cache/index0/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index1/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index2/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index3/shared_cpu_list: 8-15,136-143
> 
>   # echo 0 > /sys/devices/system/cpu/cpu8/online
>   # echo 1 > /sys/devices/system/cpu/cpu8/online
> 
>   # for i in /sys/devices/system/cpu/cpu8/cache/index*/shared_cpu_list; do echo -n "$i: "; cat $i; done
>     /sys/devices/system/cpu/cpu8/cache/index0/shared_cpu_list: 8
>     /sys/devices/system/cpu/cpu8/cache/index1/shared_cpu_list: 8
>     /sys/devices/system/cpu/cpu8/cache/index2/shared_cpu_list: 8
>     /sys/devices/system/cpu/cpu8/cache/index3/shared_cpu_list: 8
> 
>   # cat /sys/devices/system/cpu/cpu136/cache/index0/shared_cpu_list
>     136
> 
>   # cat /sys/devices/system/cpu/cpu136/cache/index3/shared_cpu_list
>     9-15,136-143
> 
> Clear the flag when the CPU is removed from shared_cpu_map when
> cache_shared_cpu_map_remove() is called during CPU hotplug. This will
> allow cache_shared_cpu_map_setup() to add the CPU coming back online in
> the shared_cpu_map. Set the flag again when the shared_cpu_map is setup.
> Following are results of performing the same test as described above with
> the changes:
> 
>   # for i in /sys/devices/system/cpu/cpu8/cache/index*/shared_cpu_list; do echo -n "$i: "; cat $i; done
>     /sys/devices/system/cpu/cpu8/cache/index0/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index1/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index2/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index3/shared_cpu_list: 8-15,136-143
> 
>   # echo 0 > /sys/devices/system/cpu/cpu8/online
>   # echo 1 > /sys/devices/system/cpu/cpu8/online
> 
>   # for i in /sys/devices/system/cpu/cpu8/cache/index*/shared_cpu_list; do echo -n "$i: "; cat $i; done
>     /sys/devices/system/cpu/cpu8/cache/index0/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index1/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index2/shared_cpu_list: 8,136
>     /sys/devices/system/cpu/cpu8/cache/index3/shared_cpu_list: 8-15,136-143
> 
>   # cat /sys/devices/system/cpu/cpu136/cache/index0/shared_cpu_list
>     8,136
> 
>   # cat /sys/devices/system/cpu/cpu136/cache/index3/shared_cpu_list
>     8-15,136-143
> 
> Fixes: 5c2712387d48 ("cacheinfo: Fix LLC is not exported through sysfs")

It's ok for me to have this tag but I don't think this is the root cause,
the commit happens to expose the problem. Other arthitectures like arm64
never updates the this_cpu_ci->cpu_map_populated even after the cpumap is
populated.

> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

Thanks for fixing this!

Reviewed-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>

> ---
>  drivers/base/cacheinfo.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index d1ae443fd7a0..cbae8be1fe52 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -410,11 +410,14 @@ static int cache_shared_cpu_map_setup(unsigned int cpu)
>  			coherency_max_size = this_leaf->coherency_line_size;
>  	}
>  
> +	/* shared_cpu_map is now populated for the cpu */
> +	this_cpu_ci->cpu_map_populated = true;
>  	return 0;
>  }
>  
>  static void cache_shared_cpu_map_remove(unsigned int cpu)
>  {
> +	struct cpu_cacheinfo *this_cpu_ci = get_cpu_cacheinfo(cpu);
>  	struct cacheinfo *this_leaf, *sib_leaf;
>  	unsigned int sibling, index, sib_index;
>  
> @@ -447,6 +450,9 @@ static void cache_shared_cpu_map_remove(unsigned int cpu)
>  			}
>  		}
>  	}
> +
> +	/* cpu is no longer populated in the shared map */
> +	this_cpu_ci->cpu_map_populated = false;
>  }
>  
>  static void free_cache_attributes(unsigned int cpu)
>