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Message-ID: <YrGQ+l8bwAsMXaX1@MiWiFi-R3L-srv>
Date: Tue, 21 Jun 2022 17:35:54 +0800
From: Baoquan He <bhe@...hat.com>
To: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
Ard Biesheuvel <ardb@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
Eric Biederman <ebiederm@...ssion.com>,
Rob Herring <robh+dt@...nel.org>,
Frank Rowand <frowand.list@...il.com>,
devicetree@...r.kernel.org, Dave Young <dyoung@...hat.com>,
Vivek Goyal <vgoyal@...hat.com>, kexec@...ts.infradead.org,
linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org,
Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
Randy Dunlap <rdunlap@...radead.org>,
Feng Zhou <zhoufeng.zf@...edance.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Chen Zhou <dingguo.cz@...group.com>,
John Donnelly <John.p.donnelly@...cle.com>,
Dave Kleikamp <dave.kleikamp@...cle.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash
high memory
On 06/21/22 at 03:56pm, Leizhen (ThunderTown) wrote:
>
>
> On 2022/6/21 13:33, Baoquan He wrote:
> > Hi,
> >
> > On 06/13/22 at 04:09pm, Zhen Lei wrote:
> >> If the crashkernel has both high memory above DMA zones and low memory
> >> in DMA zones, kexec always loads the content such as Image and dtb to the
> >> high memory instead of the low memory. This means that only high memory
> >> requires write protection based on page-level mapping. The allocation of
> >> high memory does not depend on the DMA boundary. So we can reserve the
> >> high memory first even if the crashkernel reservation is deferred.
> >>
> >> This means that the block mapping can still be performed on other kernel
> >> linear address spaces, the TLB miss rate can be reduced and the system
> >> performance will be improved.
> >
> > Ugh, this looks a little ugly, honestly.
> >
> > If that's for sure arm64 can't split large page mapping of linear
> > region, this patch is one way to optimize linear mapping. Given kdump
> > setting is necessary on arm64 server, the booting speed is truly
> > impacted heavily.
>
> There is also a performance impact when running.
Yes, indeed, the TLB flush will happen more often.
>
> >
> > However, I would suggest letting it as is with below reasons:
> >
> > 1) The code will complicate the crashkernel reservatoin code which
> > is already difficult to understand.
>
> Yeah, I feel it, too.
>
> > 2) It can only optimize the two cases, first is CONFIG_ZONE_DMA|DMA32
> > disabled, the other is crashkernel=,high is specified. While both
> > two cases are corner case, most of systems have CONFIG_ZONE_DMA|DMA32
> > enabled, and most of systems have crashkernel=xM which is enough.
> > Having them optimized won't bring benefit to most of systems.
>
> The case of CONFIG_ZONE_DMA|DMA32 disabled have been resolved by
> commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for platforms with no DMA memory zones").
> Currently the performance problem to be optimized is that DMA is enabled.
Yes, the disabled CONFIG_ZONE_DMA|DMA32 case has avoided the problem since
its boundary is decided already at that time. Crashkenrel=,high can slso
avoid this benefitting from the top done memblock allocating. However,
the crashkerne=xM which now gets the fallback support is the main syntax
we will use, that still has the problem.
>
>
> > 3) Besides, the crashkernel=,high can be handled earlier because
> > arm64 alwasys have memblock.bottom_up == false currently, thus we
> > don't need worry arbout the lower limit of crashkernel,high
> > reservation for now. If memblock.bottom_up is set true in the future,
> > this patch doesn't work any more.
> >
> >
> > ...
> > crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> > crash_base, crash_max);
> >
> > So, in my opinion, we can leave the current NON_BLOCK|SECT mapping as
> > is caused by crashkernel reserving, since no regression is brought.
> > And meantime, turning to check if there's any way to make the contiguous
> > linear mapping and later splitting work. The patch 4, 5 in this patchset
> > doesn't make much sense to me, frankly speaking.
>
> OK. As discussed earlier, I can rethink if there is a better way to patch 4-5,
> and this time focus on patch 1-2. In this way, all the functions are complete,
> and only optimization is left.
Sounds nice, thx.
> >
> >>
> >> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> >> ---
> >> arch/arm64/mm/init.c | 71 ++++++++++++++++++++++++++++++++++++++++----
> >> 1 file changed, 65 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >> index fb24efbc46f5ef4..ae0bae2cafe6ab0 100644
> >> --- a/arch/arm64/mm/init.c
> >> +++ b/arch/arm64/mm/init.c
> >> @@ -141,15 +141,44 @@ static void __init reserve_crashkernel(int dma_state)
> >> unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
> >> char *cmdline = boot_command_line;
> >> int dma_enabled = IS_ENABLED(CONFIG_ZONE_DMA) || IS_ENABLED(CONFIG_ZONE_DMA32);
> >> - int ret;
> >> + int ret, skip_res = 0, skip_low_res = 0;
> >> bool fixed_base;
> >>
> >> if (!IS_ENABLED(CONFIG_KEXEC_CORE))
> >> return;
> >>
> >> - if ((!dma_enabled && (dma_state != DMA_PHYS_LIMIT_UNKNOWN)) ||
> >> - (dma_enabled && (dma_state != DMA_PHYS_LIMIT_KNOWN)))
> >> - return;
> >> + /*
> >> + * In the following table:
> >> + * X,high means crashkernel=X,high
> >> + * unknown means dma_state = DMA_PHYS_LIMIT_UNKNOWN
> >> + * known means dma_state = DMA_PHYS_LIMIT_KNOWN
> >> + *
> >> + * The first two columns indicate the status, and the last two
> >> + * columns indicate the phase in which crash high or low memory
> >> + * needs to be reserved.
> >> + * ---------------------------------------------------
> >> + * | DMA enabled | X,high used | unknown | known |
> >> + * ---------------------------------------------------
> >> + * | N N | low | NOP |
> >> + * | Y N | NOP | low |
> >> + * | N Y | high/low | NOP |
> >> + * | Y Y | high | low |
> >> + * ---------------------------------------------------
> >> + *
> >> + * But in this function, the crash high memory allocation of
> >> + * crashkernel=Y,high and the crash low memory allocation of
> >> + * crashkernel=X[@offset] for crashk_res are mixed at one place.
> >> + * So the table above need to be adjusted as below:
> >> + * ---------------------------------------------------
> >> + * | DMA enabled | X,high used | unknown | known |
> >> + * ---------------------------------------------------
> >> + * | N N | res | NOP |
> >> + * | Y N | NOP | res |
> >> + * | N Y |res/low_res| NOP |
> >> + * | Y Y | res | low_res |
> >> + * ---------------------------------------------------
> >> + *
> >> + */
> >>
> >> /* crashkernel=X[@offset] */
> >> ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),
> >> @@ -169,10 +198,33 @@ static void __init reserve_crashkernel(int dma_state)
> >> else if (ret)
> >> return;
> >>
> >> + /* See the third row of the second table above, NOP */
> >> + if (!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN))
> >> + return;
> >> +
> >> + /* See the fourth row of the second table above */
> >> + if (dma_enabled) {
> >> + if (dma_state == DMA_PHYS_LIMIT_UNKNOWN)
> >> + skip_low_res = 1;
> >> + else
> >> + skip_res = 1;
> >> + }
> >> +
> >> crash_max = CRASH_ADDR_HIGH_MAX;
> >> } else if (ret || !crash_size) {
> >> /* The specified value is invalid */
> >> return;
> >> + } else {
> >> + /* See the 1-2 rows of the second table above, NOP */
> >> + if ((!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN)) ||
> >> + (dma_enabled && (dma_state == DMA_PHYS_LIMIT_UNKNOWN)))
> >> + return;
> >> + }
> >> +
> >> + if (skip_res) {
> >> + crash_base = crashk_res.start;
> >> + crash_size = crashk_res.end - crashk_res.start + 1;
> >> + goto check_low;
> >> }
> >>
> >> fixed_base = !!crash_base;
> >> @@ -202,9 +254,18 @@ static void __init reserve_crashkernel(int dma_state)
> >> return;
> >> }
> >>
> >> + crashk_res.start = crash_base;
> >> + crashk_res.end = crash_base + crash_size - 1;
> >> +
> >> +check_low:
> >> + if (skip_low_res)
> >> + return;
> >> +
> >> if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
> >> crash_low_size && reserve_crashkernel_low(crash_low_size)) {
> >> memblock_phys_free(crash_base, crash_size);
> >> + crashk_res.start = 0;
> >> + crashk_res.end = 0;
> >> return;
> >> }
> >>
> >> @@ -219,8 +280,6 @@ static void __init reserve_crashkernel(int dma_state)
> >> if (crashk_low_res.end)
> >> kmemleak_ignore_phys(crashk_low_res.start);
> >>
> >> - crashk_res.start = crash_base;
> >> - crashk_res.end = crash_base + crash_size - 1;
> >> insert_resource(&iomem_resource, &crashk_res);
> >> }
> >>
> >> --
> >> 2.25.1
> >>
> >
> > .
> >
>
> --
> Regards,
> Zhen Lei
>
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