[PATCH v3] powerpc/pseries: explicitly reschedule during drmem_lmb list traversal
Christophe Leroy
christophe.leroy at csgroup.eu
Fri Aug 14 15:00:51 AEST 2020
Le 13/08/2020 à 17:11, 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.
>
> Introduce a drmem_lmb_next() iteration helper function which calls
> cond_resched() at a regular interval during array traversal. Each
> iteration of the loop in DLPAR code paths can involve around ten RTAS
> calls which can each take up to 250us, so this ensures the check is
> performed at worst every few milliseconds.
>
> Fixes: 6c6ea53725b3 ("powerpc/mm: Separate ibm, dynamic-memory data from DT format")
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
Looks a lot better to me than v2.
Reviewed-by: Christophe Leroy <christophe.leroy at csgroup.eu>
> ---
>
> Notes:
> Changes since v2:
> * Make drmem_lmb_next() more general.
> * Adjust reschedule interval for better code generation.
> * Add commentary to drmem_lmb_next() to explain the cond_resched()
> call.
> * Remove bounds assertions.
>
> Changes since v1:
> * Add bounds assertions in drmem_lmb_next().
> * Call cond_resched() in the iterator on only every 20th element
> instead of on every iteration, to reduce overhead in tight loops.
>
> arch/powerpc/include/asm/drmem.h | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/drmem.h b/arch/powerpc/include/asm/drmem.h
> index 17ccc6474ab6..6fb928605ed1 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 +28,22 @@ struct drmem_lmb_info {
>
> extern struct drmem_lmb_info *drmem_info;
>
> +static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb,
> + const struct drmem_lmb *start)
> +{
> + /*
> + * DLPAR code paths can take several milliseconds per element
> + * when interacting with firmware. Ensure that we don't
> + * unfairly monopolize the CPU.
> + */
> + if (((++lmb - start) % 16) == 0)
> + 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, start))
>
> #define for_each_drmem_lmb(lmb) \
> for_each_drmem_lmb_in_range((lmb), \
>
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