linux-kernel - Re: [PATCH] irqchip/gic-v3-its: add allocation max order limitation for lpi_id

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Message-ID: <867ekcxjj5.wl-marc.zyngier@arm.com>
Date:   Mon, 27 Aug 2018 10:09:50 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Jia He <hejianet@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Jia He <jia.he@...-semitech.com>
Subject: Re: [PATCH] irqchip/gic-v3-its: add allocation max order limitation for lpi_id_bits

On Mon, 27 Aug 2018 02:41:04 +0100,
Jia He <hejianet@...il.com> wrote:
> 
> Hi Marc
> Thanks for the comments
> 
> On 8/27/2018 3:01 AM, Marc Zyngier Wrote:
> > [I'm travelling, so expect some major delays in responding to email]
> > 
> > Hi Jia,
> > 
> > On Sun, 26 Aug 2018 10:00:51 +0100,
> > Jia He <hejianet@...il.com> wrote:
> >>
> >> There is a WARN_ON when my QDF2400 server boots up (pagesize is 4k)
> > 
> > [snip]
> > 
> >> In its_alloc_lpi_tables, lpi_id_bits is 24, without this patch,
> >> its_allocate_prop_table will try to allocate 16M(order 12 if
> >> pagesize=4k). Thus it causes the WARN_ON.
> > 
> > Gah! QDF and its 24bit INTIDs... Making life hell for everyone ;-)
> > 
> > Sorry for breaking it.
> 
> np, maybe QDF2400 is a little bit special
> 
> > 
> >>
> >> This patch fixes it by limiting the lpi_id_bits.
> >>
> >> Signed-off-by: Jia He <jia.he@...-semitech.com>
> >> ---
> >>  drivers/irqchip/irq-gic-v3-its.c | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> >> index 316a575..79e6993 100644
> >> --- a/drivers/irqchip/irq-gic-v3-its.c
> >> +++ b/drivers/irqchip/irq-gic-v3-its.c
> >> @@ -1624,8 +1624,11 @@ static void its_free_prop_table(struct page *prop_page)
> >>  static int __init its_alloc_lpi_tables(void)
> >>  {
> >>  	phys_addr_t paddr;
> >> +	u32 max_bits;	 /*max order limitation in alloc_page*/
> >>  
> >> -	lpi_id_bits = GICD_TYPER_ID_BITS(gic_rdists->gicd_typer);
> >> +	max_bits = PAGE_SHIFT + MAX_ORDER - 1;
> >> +	lpi_id_bits = min_t(u32, max_bits,
> >> +			GICD_TYPER_ID_BITS(gic_rdists->gicd_typer));
> >>  	gic_rdists->prop_page = its_allocate_prop_table(GFP_NOWAIT);
> >>  	if (!gic_rdists->prop_page) {
> >>  		pr_err("Failed to allocate PROPBASE\n");
> >> -- 
> >> 1.8.3.1
> >>
> > 
> > I find it rather odd that we end-up with different interrupt ranges
> > depending on the CPU page size. Also, allocating that much memory for
> > LPIs is rather pointless, as we actually have a pretty low limit of
> > interrupts the system can deal with (see IRQ_BITMAP_BITS, which is
> > slightly more than 8k). I've so far seen *one* request to push it up,
> > but I doubt that it is a real use case.
> > 
> > Capping lpi_id_bits at 16 (which is what we had before) is plenty,
> > will save a some memory, and gives some margin before we need to push
> > it up again.
> 
> Do you want me to revert commit fe8e93504 to cap the lpi_id_bits
> to no greater than ITS_MAX_LPI_NRBITS(16) instead this patch?

Yes, this is probably the best course of action at this time. When I
dropped this, I was mostly thinking of the LPI allocator (which now
works with an almost constant memory footprint), and didn't pay enough
attention to the property table allocation (which is still a function
of the number of LPIs).

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.