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References: <20190926045419.22827-1-alastair@au1.ibm.com> <20190926045419.22827-6-alastair@au1.ibm.com>
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From:   Mark Marshall <markmarshall14@gmail.com>
Date:   Fri, 27 Sep 2019 08:37:56 +0200
Message-ID: <CAD4b4WLdMgunRoBDVtNZbhaMPbMw57AbcJgkKfmy2zpshgVyqQ@mail.gmail.com>
Subject: Re: [PATCH v4 5/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory
To:     "Alastair D'Silva" <alastair@au1.ibm.com>
Cc:     alastair@d-silva.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Qian Cai <cai@lca.pw>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        Allison Randal <allison@lohutok.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
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Comment below...

On Thu, 26 Sep 2019 at 12:18, Alastair D'Silva <alastair@au1.ibm.com> wrote:
>
> From: Alastair D'Silva <alastair@d-silva.org>
>
> When presented with large amounts of memory being hotplugged
> (in my test case, ~890GB), the call to flush_dcache_range takes
> a while (~50 seconds), triggering RCU stalls.
>
> This patch breaks up the call into 1GB chunks, calling
> cond_resched() inbetween to allow the scheduler to run.
>
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>  arch/powerpc/mm/mem.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index cff947cb2a84..a2758e473d58 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -104,6 +104,27 @@ int __weak remove_section_mapping(unsigned long start, unsigned long end)
>         return -ENODEV;
>  }
>
> +#define FLUSH_CHUNK_SIZE SZ_1G
> +/**
> + * flush_dcache_range_chunked(): Write any modified data cache blocks out to
> + * memory and invalidate them, in chunks of up to FLUSH_CHUNK_SIZE
> + * Does not invalidate the corresponding instruction cache blocks.
> + *
> + * @start: the start address
> + * @stop: the stop address (exclusive)
> + * @chunk: the max size of the chunks
> + */
> +static void flush_dcache_range_chunked(unsigned long start, unsigned long stop,
> +                                      unsigned long chunk)
> +{
> +       unsigned long i;
> +
> +       for (i = start; i < stop; i += FLUSH_CHUNK_SIZE) {
Here you ignore the function parameter "chunk" and use the define
FLUSH_CHUNK_SIZE.
You should do one or the other; use the parameter or remove it.
> +               flush_dcache_range(i, min(stop, start + FLUSH_CHUNK_SIZE));
> +               cond_resched();
> +       }
> +}
> +
>  int __ref arch_add_memory(int nid, u64 start, u64 size,
>                         struct mhp_restrictions *restrictions)
>  {
> @@ -120,7 +141,8 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
>                         start, start + size, rc);
>                 return -EFAULT;
>         }
> -       flush_dcache_range(start, start + size);
> +
> +       flush_dcache_range_chunked(start, start + size, FLUSH_CHUNK_SIZE);
>
>         return __add_pages(nid, start_pfn, nr_pages, restrictions);
>  }
> @@ -137,7 +159,8 @@ void __ref arch_remove_memory(int nid, u64 start, u64 size,
>
>         /* Remove htab bolted mappings for this section of memory */
>         start = (unsigned long)__va(start);
> -       flush_dcache_range(start, start + size);
> +       flush_dcache_range_chunked(start, start + size, FLUSH_CHUNK_SIZE);
> +
>         ret = remove_section_mapping(start, start + size);
>         WARN_ON_ONCE(ret);
>
> --
> 2.21.0
>