linux-kernel - Re: [PATCH v3] usb: gadget: aspeed: improve vhub port irq handling

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Date:   Tue, 7 Apr 2020 22:40:23 -0700
From:   Tao Ren <rentao.bupt@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Felipe Balbi <balbi@...nel.org>, linux-aspeed@...ts.ozlabs.org,
        Andrew Jeffery <andrew@...id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        openbmc@...ts.ozlabs.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        Joel Stanley <joel@....id.au>, taoren@...com,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3] usb: gadget: aspeed: improve vhub port irq handling

On Wed, Apr 08, 2020 at 09:36:16AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2020-04-06 at 23:02 -0700, Tao Ren wrote:
> > I ran some testing on my ast2400 and ast2500 BMC and looks like the
> > for() loop runs faster than for_each_set_bit_from() loop in my
> > environment. I'm not sure if something needs to be revised in my test
> > code, but please kindly share your suggestions:
> > 
> > I use get_cycles() to calculate execution time of 2 different loops, and
> > ast_vhub_dev_irq() is replaced with barrier() to avoid "noise"; below
> > are the results:
> > 
> >   - when downstream port number is 5 and only 1 irq bit is set, it takes
> >     ~30 cycles to finish for_each_set_bit() loop, and 20-25 cycles to
> >     finish the for() loop.
> > 
> >   - if downstream port number is 5 and all 5 bits are set, then
> >     for_each_set_bit() loop takes ~50 cycles and for() loop takes ~25
> >     cycles.
> > 
> >   - when I increase downsteam port number to 16 and set 1 irq bit, the
> >     for_each_set_bit() loop takes ~30 cycles and for() loop takes 25
> >     cycles. It's a little surprise to me because I thought for() loop
> >     would cost 60+ cycles (3 times of the value when port number is 5).
> > 
> >   - if downstream port number is 16 and all irq status bits are set,
> >     then for_each_set_bit() loop takes 60-70 cycles and for() loop takes
> >     30+ cycles.
> 
> I suspect the CPU doesn't have an efficient find-zero-bit primitive,
> check the generated asm. In that case I would go back to the simple for
> loop.
> 
> Cheers,
> Ben.

_find_next_bit_le() function is defined in arch/arm/lib/findbit.S. I'm
looking at the code: will run more tests and send out patch v4 with
simple for loop later.


Cheers,

Tao