Re: [PATCH v2] powerpc/mm: Add cond_resched() while removing hpte mappings

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

Le 04/04/2021 à 18:31, 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. The
> call to cond_resched() is issued every HZ jiffies which should prevent
> the soft-lockup from being reported.
> 
> Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> 
> ---
> Changelog:
> 
> v2: Issue cond_resched() every HZ jiffies instead of each iteration of
>      the loop. [ Christophe Leroy ]
> ---
>   arch/powerpc/mm/book3s64/hash_utils.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index 581b20a2feaf..286e7e8cb919 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -338,7 +338,7 @@ int htab_bolt_mapping(unsigned long vstart, unsigned long vend,
>   int htab_remove_mapping(unsigned long vstart, unsigned long vend,
>   		      int psize, int ssize)
>   {
> -	unsigned long vaddr;
> +	unsigned long vaddr, time_limit;
>   	unsigned int step, shift;
>   	int rc;
>   	int ret = 0;
> @@ -351,8 +351,19 @@ int htab_remove_mapping(unsigned long vstart, unsigned long vend,
>   
>   	/* Unmap the full range specificied */
>   	vaddr = ALIGN_DOWN(vstart, step);
> +	time_limit = jiffies + HZ;
> +
>   	for (;vaddr < vend; vaddr += step) {
>   		rc = mmu_hash_ops.hpte_removebolted(vaddr, psize, ssize);
> +
> +		/*
> +		 * For large number of mappings introduce a cond_resched()
> +		 * to prevent softlockup warnings.
> +		 */
> +		if (time_after(jiffies, time_limit)) {
> +			cond_resched();
> +			time_limit = jiffies + HZ;
> +		}
>   		if (rc == -ENOENT) {
>   			ret = -ENOENT;
>   			continue;
> 
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