Re: [PATCH] powerpc/pseries: explicitly reschedule during drmem_lmb list traversal

[Laurent Dufour <ldufour@linux.ibm.com>]

Le 28/07/2020 à 19:37, Nathan Lynch a écrit :
> The drmem lmb list can have hundreds of thousands of entries, and
> unfortunately lookups take the form of linear searches. As long as
> this is the case, traversals have the potential to monopolize the CPU
> and provoke lockup reports, workqueue stalls, and the like unless
> they explicitly yield.
> 
> Rather than placing cond_resched() calls within various
> for_each_drmem_lmb() loop blocks in the code, put it in the iteration
> expression of the loop macro itself so users can't omit it.

Hi Nathan,

Is that not too much to call cond_resched() on every LMB?

Could that be less frequent, every 10, or 100, I don't really know ?

Cheers,
Laurent.
> 
> Fixes: 6c6ea53725b3 ("powerpc/mm: Separate ibm, dynamic-memory data from DT format")
> Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/drmem.h | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/drmem.h b/arch/powerpc/include/asm/drmem.h
> index 414d209f45bb..36d0ed04bda8 100644
> --- a/arch/powerpc/include/asm/drmem.h
> +++ b/arch/powerpc/include/asm/drmem.h
> @@ -8,6 +8,8 @@
>   #ifndef _ASM_POWERPC_LMB_H
>   #define _ASM_POWERPC_LMB_H
>   
> +#include <linux/sched.h>
> +
>   struct drmem_lmb {
>   	u64     base_addr;
>   	u32     drc_index;
> @@ -26,8 +28,14 @@ struct drmem_lmb_info {
>   
>   extern struct drmem_lmb_info *drmem_info;
>   
> +static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb)
> +{
> +	cond_resched();
> +	return ++lmb;
> +}
> +
>   #define for_each_drmem_lmb_in_range(lmb, start, end)		\
> -	for ((lmb) = (start); (lmb) < (end); (lmb)++)
> +	for ((lmb) = (start); (lmb) < (end); lmb = drmem_lmb_next(lmb))
>   
>   #define for_each_drmem_lmb(lmb)					\
>   	for_each_drmem_lmb_in_range((lmb),			\
>