Re: [PATCH 6.12.y-cip] mm: page_alloc: Add kernel parameter to select maximum PCP batch scale number

[Pavel Machek <pavel@denx.de>]

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Hi!

> Commit 52166607ecc9 ("mm: restrict the pcp batch scale factor to avoid
> too long latency") introduced default PCP (Per-CPU Pageset) batch size as
> a configuration flag. The configuration flag is CONFIG_PCP_BATCH_SCALE_MAX.
> 
> The ARM64 defconfig has CONFIG_PCP_BATCH_SCALE_MAX=5. This defconfig
> is used by a high range of SoCs.
> 
> The Renesas RZ/G3S SoC is a single CPU SoC, with L1$ (I-cache 32Kbytes,
> D-cache 32 Kbytes), L3$ (256 Kbytes), but no L2$. It is currently used in
> a configuration with 1 GiB RAM size. In this configuration, starting with
> commit 52166607ecc9 ("mm: restrict the pcp batch scale factor to avoid too
> long latency") the "bonnie++ -d /mnt -u root" benchmark takes ~14 minutes
> while previously it took ~10 minutes. The /mnt directory is mounted on SD
> card. Same behavior is reproduced on similar Renesas single core devices
> (e.g., Renesas RZ/G2UL). bonnie++ version used was 1.04.
> 
> Add a new kernel parameter to allow systems like Renesas RZ/G3S to
> continue have the same performance numbers with the default mainline
...

> This patch was posted as RFC at [1] but got no feedback. Starting with
> kernel v6.17 the behaviour was restored by commits:
> 
> 18ebe55a9236 ("mm/readahead: terminate async readahead on natural boundary")
> 38b0ece6d763 ("mm/filemap: allow arch to request folio size for exec
> memory")

Ok, so this is not mainline, because mainline has different solution.

It is also not queued for stable.

It introduces kernel parameter that will be special for -cip, not seen
in mainline nor -stable.

Presumably you'll be telling your customers to set the command line
parameter. Would it make sense to ask them to tune the config instead?

Best regards,
									Pavel

> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4734,6 +4734,12 @@
>  			for debug and development, but should not be
>  			needed on a platform with proper driver support.
>  
> +	pcp_batch_scale_max=n
> +			Format: <integer>
> +			Range: 0,6 : number
> +			Default : CONFIG_PCP_BATCH_SCALE_MAX
> +			Used for setting the scale number for PCP batch scale algorithm.
> +
>  	pdcchassis=	[PARISC,HW] Disable/Enable PDC Chassis Status codes at
>  			boot time.
>  			Format: { 0 | 1 }

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