Re: [PATCH] powerpc/mm: Add cond_resched() while removing hpte mappings

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 10/03/2021 à 08:59, Vaibhav Jain a écrit :
> While removing large number of mappings from hash page tables for
> large memory systems as soft-lockup is reported because of the time
> spent inside htap_remove_mapping() like one below:
> 
>   watchdog: BUG: soft lockup - CPU#8 stuck for 23s!
>   <snip>
>   NIP plpar_hcall+0x38/0x58
>   LR  pSeries_lpar_hpte_invalidate+0x68/0xb0
>   Call Trace:
>    0x1fffffffffff000 (unreliable)
>    pSeries_lpar_hpte_removebolted+0x9c/0x230
>    hash__remove_section_mapping+0xec/0x1c0
>    remove_section_mapping+0x28/0x3c
>    arch_remove_memory+0xfc/0x150
>    devm_memremap_pages_release+0x180/0x2f0
>    devm_action_release+0x30/0x50
>    release_nodes+0x28c/0x300
>    device_release_driver_internal+0x16c/0x280
>    unbind_store+0x124/0x170
>    drv_attr_store+0x44/0x60
>    sysfs_kf_write+0x64/0x90
>    kernfs_fop_write+0x1b0/0x290
>    __vfs_write+0x3c/0x70
>    vfs_write+0xd4/0x270
>    ksys_write+0xdc/0x130
>    system_call+0x5c/0x70
> 
> Fix this by adding a cond_resched() to the loop in
> htap_remove_mapping() that issues hcall to remove hpte mapping. This
> should prevent the soft-lockup from being reported.

Isn't it overkill to call is at each iteration ?

Looking at a few other places, there is some mitigation. For instance fadump_free_reserved_memory() 
does it based on elapsed time. Another exemple is drmem_lmb_next() doing it every 16 iteration.


> 
> Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> ---
>   arch/powerpc/mm/book3s64/hash_utils.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index 581b20a2feaf..ea3945c70b18 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -359,6 +359,8 @@ int htab_remove_mapping(unsigned long vstart, unsigned long vend,
>   		}
>   		if (rc < 0)
>   			return rc;
> +
> +		cond_resched();
>   	}
>   
>   	return ret;
> 

Christophe