linux-kernel - Re: [PATCH] powerpc: pci-ioda: Optimize pnv_ioda_pick_m64

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Message-ID: <874a2b30-ec4d-4b72-9f25-81429f03ccb7@kernel.org>
Date: Tue, 22 Jul 2025 10:36:05 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Yury Norov <yury.norov@...il.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Thomas Gleixner <tglx@...utronix.de>, Frederic Barrat
 <fbarrat@...ux.ibm.com>, Andrew Donnellan <ajd@...ux.ibm.com>,
 linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: pci-ioda: Optimize pnv_ioda_pick_m64_pe()

On 20. 07. 25, 3:05, Yury Norov wrote:
> bitmap_empty() in pnv_ioda_pick_m64_pe() is O(N) and useless, because
> the following find_next_bit() does the same work.
> 
> Drop it, and while there replace a while() loop with the dedicated
> for_each_set_bit().
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>   arch/powerpc/platforms/powernv/pci-ioda.c | 18 +++++-------------
>   1 file changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
> index d8ccf2c9b98a..c7a421ead992 100644
> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> @@ -313,24 +313,16 @@ static struct pnv_ioda_pe *pnv_ioda_pick_m64_pe(struct pci_bus *bus, bool all)
>   	/* Figure out reserved PE numbers by the PE */
>   	pnv_ioda_reserve_m64_pe(bus, pe_alloc, all);
>   
> -	/*
> -	 * the current bus might not own M64 window and that's all
> -	 * contributed by its child buses. For the case, we needn't
> -	 * pick M64 dependent PE#.
> -	 */
> -	if (bitmap_empty(pe_alloc, phb->ioda.total_pe_num)) {
> -		kfree(pe_alloc);
> -		return NULL;
> -	}
> -
>   	/*
>   	 * Figure out the master PE and put all slave PEs to master
>   	 * PE's list to form compound PE.
> +	 *
> +	 * The current bus might not own M64 window and that's all
> +	 * contributed by its child buses. For the case, we needn't
> +	 * pick M64 dependent PE#.
>   	 */
>   	master_pe = NULL;
> -	i = -1;
> -	while ((i = find_next_bit(pe_alloc, phb->ioda.total_pe_num, i + 1)) <
> -		phb->ioda.total_pe_num) {
> +	for_each_set_bit(i, pe_alloc, phb->ioda.total_pe_num) {

Makes sense. Could you also:
* use bitmap_alloc()
* use __cleanup() to free the bitmap
* make i unsigned
?

thanks,
-- 
js
suse labs