LinuxPPC-Dev Archive on lore.kernel.org
 help / color / Atom feed
From: David Gibson <david@gibson.dropbear.id.au>
To: Leonardo Bras <leobras.c@gmail.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Scott Cheloha <cheloha@linux.ibm.com>,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Nicholas Piggin <npiggin@gmail.com>,
	Bharata B Rao <bharata@linux.ibm.com>,
	Paul Mackerras <paulus@samba.org>,
	Sandipan Das <sandipan@linux.ibm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Laurent Dufour <ldufour@linux.ibm.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Mike Rapoport <rppt@kernel.org>
Subject: Re: [PATCH 2/3] powerpc/mm/hash: Avoid multiple HPT resize-ups on memory hotplug
Date: Mon, 22 Mar 2021 18:55:35 +1100
Message-ID: <YFhNd42RvobCV8tF@yekko.fritz.box> (raw)
In-Reply-To: <20210312072940.598696-3-leobras.c@gmail.com>


[-- Attachment #1: Type: text/plain, Size: 6496 bytes --]

On Fri, Mar 12, 2021 at 04:29:40AM -0300, Leonardo Bras wrote:
> Every time a memory hotplug happens, and the memory limit crosses a 2^n
> value, it may be necessary to perform HPT resizing-up, which can take
> some time (over 100ms in my tests).
> 
> It usually is not an issue, but it can take some time if a lot of memory
> is added to a guest with little starting memory:
> Adding 256G to a 2GB guest, for example will require 8 HPT resizes.
> 
> Perform an HPT resize before memory hotplug, updating HPT to its
> final size (considering a successful hotplug), taking the number of
> HPT resizes to at most one per memory hotplug action.
> 
> Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
> ---
>  arch/powerpc/include/asm/book3s/64/hash.h       |  2 ++
>  arch/powerpc/include/asm/sparsemem.h            |  2 ++
>  arch/powerpc/mm/book3s64/hash_utils.c           | 14 ++++++++++++++
>  arch/powerpc/mm/book3s64/pgtable.c              |  6 ++++++
>  arch/powerpc/platforms/pseries/hotplug-memory.c |  6 ++++++
>  5 files changed, 30 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
> index d959b0195ad9..843b0a178590 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash.h
> @@ -255,6 +255,8 @@ int hash__create_section_mapping(unsigned long start, unsigned long end,
>  				 int nid, pgprot_t prot);
>  int hash__remove_section_mapping(unsigned long start, unsigned long end);
>  
> +void hash_memory_batch_expand_prepare(unsigned long newsize);
> +
>  #endif /* !__ASSEMBLY__ */
>  #endif /* __KERNEL__ */
>  #endif /* _ASM_POWERPC_BOOK3S_64_HASH_H */
> diff --git a/arch/powerpc/include/asm/sparsemem.h b/arch/powerpc/include/asm/sparsemem.h
> index d072866842e4..16b5f5300c84 100644
> --- a/arch/powerpc/include/asm/sparsemem.h
> +++ b/arch/powerpc/include/asm/sparsemem.h
> @@ -17,6 +17,8 @@ extern int remove_section_mapping(unsigned long start, unsigned long end);
>  extern int memory_add_physaddr_to_nid(u64 start);
>  #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
>  
> +void memory_batch_expand_prepare(unsigned long newsize);
> +
>  #ifdef CONFIG_NUMA
>  extern int hot_add_scn_to_nid(unsigned long scn_addr);
>  #else
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index cfb3ec164f56..1f6aa0bf27e7 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -858,6 +858,20 @@ int hash__remove_section_mapping(unsigned long start, unsigned long end)
>  
>  	return rc;
>  }
> +
> +void hash_memory_batch_expand_prepare(unsigned long newsize)
> +{
> +	/*
> +	 * Resizing-up HPT should never fail, but there are some cases system starts with higher
> +	 * SHIFT than required, and we go through the funny case of resizing HPT down while
> +	 * adding memory
> +	 */
> +
> +	while (resize_hpt_for_hotplug(newsize, false) == -ENOSPC) {
> +		newsize *= 2;
> +		pr_warn("Hash collision while resizing HPT\n");

This unbounded increase in newsize makes me nervous - we should be
bounded by the current size of the HPT at least.  In practice we
should be fine, since the resize should always succeed by the time we
reach our current HPT size, but that's far from obvious from this
point in the code.

And... you're doubling newsize which is a value which might not be a
power of 2.  I'm wondering if there's an edge case where this could
actually cause us to skip the current size and erroneously resize to
one bigger than we have currently.

> +	}
> +}
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  
>  static void __init hash_init_partition_table(phys_addr_t hash_table,
> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> index 5b3a3bae21aa..f1cd8af0f67f 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -193,6 +193,12 @@ int __meminit remove_section_mapping(unsigned long start, unsigned long end)
>  
>  	return hash__remove_section_mapping(start, end);
>  }
> +
> +void memory_batch_expand_prepare(unsigned long newsize)

This wrapper doesn't seem useful.

> +{
> +	if (!radix_enabled())
> +		hash_memory_batch_expand_prepare(newsize);
> +}
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  
>  void __init mmu_partition_table_init(void)
> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> index 8377f1f7c78e..353c71249214 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> @@ -671,6 +671,8 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add)
>  	if (lmbs_available < lmbs_to_add)
>  		return -EINVAL;
>  
> +	memory_batch_expand_prepare(memblock_phys_mem_size() + lmbs_to_add * drmem_lmb_size());
> +
>  	for_each_drmem_lmb(lmb) {
>  		if (lmb->flags & DRCONF_MEM_ASSIGNED)
>  			continue;
> @@ -734,6 +736,8 @@ static int dlpar_memory_add_by_index(u32 drc_index)
>  
>  	pr_info("Attempting to hot-add LMB, drc index %x\n", drc_index);
>  
> +	memory_batch_expand_prepare(memblock_phys_mem_size() +
> +				     drmem_info->n_lmbs * drmem_lmb_size());

This doesn't look right.  memory_add_by_index() is adding a *single*
LMB, I think using drmem_info->n_lmbs here means you're counting this
as adding again as much memory as you already have hotplugged.

>  	lmb_found = 0;
>  	for_each_drmem_lmb(lmb) {
>  		if (lmb->drc_index == drc_index) {
> @@ -788,6 +792,8 @@ static int dlpar_memory_add_by_ic(u32 lmbs_to_add, u32 drc_index)
>  	if (lmbs_available < lmbs_to_add)
>  		return -EINVAL;
>  
> +	memory_batch_expand_prepare(memblock_phys_mem_size() + lmbs_to_add * drmem_lmb_size());
> +
>  	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
>  		if (lmb->flags & DRCONF_MEM_ASSIGNED)
>  			continue;

I don't see memory_batch_expand_prepare() suppressing any existing HPT
resizes.  Won't this just resize to the right size for the full add,
then resize several times again as we perform the add?  Or.. I guess
that will be suppressed by patch 1/3.  That's seems kinda fragile,
though.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-12  7:29 [PATCH 0/3] powerpc/mm/hash: Time improvements for memory hot(un)plug Leonardo Bras
2021-03-12  7:29 ` [PATCH 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug Leonardo Bras
2021-03-22  6:49   ` David Gibson
2021-04-09  2:16     ` Leonardo Bras
2021-03-12  7:29 ` [PATCH 2/3] powerpc/mm/hash: Avoid multiple HPT resize-ups on " Leonardo Bras
2021-03-22  7:55   ` David Gibson [this message]
2021-04-09  2:51     ` Leonardo Bras
2021-04-19  5:34       ` David Gibson
2021-03-12  7:29 ` [PATCH 3/3] powerpc/mm/hash: Avoid multiple HPT resize-downs on memory hotunplug Leonardo Bras
2021-03-22 23:45   ` David Gibson
2021-04-09  3:31     ` Leonardo Bras
2021-04-19  5:37       ` David Gibson

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=YFhNd42RvobCV8tF@yekko.fritz.box \
    --to=david@gibson.dropbear.id.au \
    --cc=akpm@linux-foundation.org \
    --cc=aneesh.kumar@linux.ibm.com \
    --cc=bharata@linux.ibm.com \
    --cc=cheloha@linux.ibm.com \
    --cc=dan.j.williams@intel.com \
    --cc=david@redhat.com \
    --cc=ldufour@linux.ibm.com \
    --cc=leobras.c@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=logang@deltatee.com \
    --cc=nathanl@linux.ibm.com \
    --cc=npiggin@gmail.com \
    --cc=paulus@samba.org \
    --cc=rppt@kernel.org \
    --cc=sandipan@linux.ibm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LinuxPPC-Dev Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linuxppc-dev/0 linuxppc-dev/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linuxppc-dev linuxppc-dev/ https://lore.kernel.org/linuxppc-dev \
		linuxppc-dev@lists.ozlabs.org linuxppc-dev@ozlabs.org
	public-inbox-index linuxppc-dev

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.ozlabs.lists.linuxppc-dev


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git