On 8/11/20 6:20 PM, Nathan Lynch wrote:
> 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.
>
> Call cond_resched() on every 20th element. 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 <nath...@linux.ibm.com>
> ---
> arch/powerpc/include/asm/drmem.h | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
> 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.
>
> diff --git a/arch/powerpc/include/asm/drmem.h
> b/arch/powerpc/include/asm/drmem.h
> index 17ccc6474ab6..583277e30dd2 100644
> --- a/arch/powerpc/include/asm/drmem.h
> +++ b/arch/powerpc/include/asm/drmem.h
> @@ -8,6 +8,9 @@
> #ifndef _ASM_POWERPC_LMB_H
> #define _ASM_POWERPC_LMB_H
>
> +#include <linux/bug.h>
> +#include <linux/sched.h>
> +
> struct drmem_lmb {
> u64 base_addr;
> u32 drc_index;
> @@ -26,8 +29,21 @@ struct drmem_lmb_info {
>
> extern struct drmem_lmb_info *drmem_info;
>
> +static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb)
> +{
> + const unsigned int resched_interval = 20;
> +
> + BUG_ON(lmb < drmem_info->lmbs);
> + BUG_ON(lmb >= drmem_info->lmbs + drmem_info->n_lmbs);
I think BUG_ON is a pretty big no-no these days unless there is no other option.
-Tyrel
> +
> + if ((lmb - drmem_info->lmbs) % resched_interval == 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))
>
> #define for_each_drmem_lmb(lmb) \
> for_each_drmem_lmb_in_range((lmb), \
>
