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Subject: Re: [PATCH v2] powerpc/pseries: explicitly reschedule during
 drmem_lmb list traversal
To: Nathan Lynch <nathanl@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
References: <20200812012005.1919255-1-nathanl@linux.ibm.com>
From: Christophe Leroy <christophe.leroy@csgroup.eu>
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Date: Wed, 12 Aug 2020 07:19:26 +0200
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Le 12/08/2020 à 03:20, 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.
> 
> 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 <nathanl@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);

BUG_ON() shall be avoided unless absolutely necessary.
Wouldn't WARN_ON() together with an early return be enough ?

> +
> +	if ((lmb - drmem_info->lmbs) % resched_interval == 0)
> +		cond_resched();

Do you need something that precise ? Can't you use 16 or 32 and use a 
logical AND instead of a MODULO ?

And what garanties that lmb is always an element of a table based at 
drmem_info->lmbs ?

What about:

static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb, 
struct drmem_lmb *start)
{
	const unsigned int resched_interval = 16;

	if ((++lmb - start) & resched_interval == 0)
		cond_resched();

	return lmb;
}

#define for_each_drmem_lmb_in_range(lmb, start, end)		\
	for ((lmb) = (start); (lmb) < (end); lmb = drmem_lmb_next(lmb, start))


> +
> +	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),			\
> 

Christophe