Re: [PATCH] zsmalloc: Fix TLB coherency and build problem

[Minchan Kim <minchan@kernel.org>]

On Tue, Feb 19, 2013 at 06:07:40PM +0800, Simon Jeons wrote:
> On 01/28/2013 09:00 AM, Minchan Kim wrote:
> >Recently, Matt Sealey reported he fail to build zsmalloc caused by
> >using of local_flush_tlb_kernel_range which are architecture dependent
> >function so !CONFIG_SMP in ARM couldn't implement it so it ends up
> >build error following as.
> 
> Confuse me!
> 
> 1) Why I see flush_tlb_kernel_range is different in different architecture?

IMHO, all architecture can do their best effort by thier own way.

> 2) Does local here means local cpu? If the answer is yes, why ARM

Yes.

> doesn't support it?

ARM supports it for some configuration and CPUs.
The thing is that every architecture doesn't support so it's not a generic API.
It means we should avoid it in general layer.

> 
> >
> >   MODPOST 216 modules
> >   LZMA    arch/arm/boot/compressed/piggy.lzma
> >   AS      arch/arm/boot/compressed/lib1funcs.o
> >ERROR: "v7wbi_flush_kern_tlb_range"
> >[drivers/staging/zsmalloc/zsmalloc.ko] undefined!
> >make[1]: *** [__modpost] Error 1
> >make: *** [modules] Error 2
> >make: *** Waiting for unfinished jobs....
> >
> >The reason we used that function is copy method by [1]
> >was really slow in ARM but at that time.
> >
> >More severe problem is ARM can prefetch speculatively on other CPUs
> >so under us, other TLBs can have an entry only if we do flush local
> >CPU. Russell King pointed that. Thanks!
> >We don't have many choices except using flush_tlb_kernel_range.
> >
> >My experiment in ARMv7 processor 4 core didn't make any difference with
> >zsmapbench[2] between local_flush_tlb_kernel_range and flush_tlb_kernel_range
> >but still page-table based is much better than copy-based.
> >
> >* bigger is better.
> >
> >1. local_flush_tlb_kernel_range: 3918795 mappings
> >2. flush_tlb_kernel_range : 3989538 mappings
> >3. copy-based: 635158 mappings
> >
> >This patch replace local_flush_tlb_kernel_range with
> >flush_tlb_kernel_range which are avaialbe in all architectures
> >because we already have used it in vmalloc allocator which are
> >generic one so build problem should go away and performane loss
> >shoud be void.
> >
> >[1] f553646, zsmalloc: add page table mapping method
> >[2] https://github.com/spartacus06/zsmapbench
> >
> >Cc: stable@vger.kernel.org
> >Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
> >Cc: Russell King <linux@arm.linux.org.uk>
> >Cc: Konrad Rzeszutek Wilk <konrad@darnok.org>
> >Cc: Nitin Gupta <ngupta@vflare.org>
> >Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
> >Reported-by: Matt Sealey <matt@genesi-usa.com>
> >Signed-off-by: Minchan Kim <minchan@kernel.org>
> >---
> >
> >Matt, Could you test this patch?
> >
> >  drivers/staging/zsmalloc/zsmalloc-main.c |   10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> >diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> >index eb00772..82e627c 100644
> >--- a/drivers/staging/zsmalloc/zsmalloc-main.c
> >+++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> >@@ -222,11 +222,9 @@ struct zs_pool {
> >  /*
> >   * By default, zsmalloc uses a copy-based object mapping method to access
> >   * allocations that span two pages. However, if a particular architecture
> >- * 1) Implements local_flush_tlb_kernel_range() and 2) Performs VM mapping
> >- * faster than copying, then it should be added here so that
> >- * USE_PGTABLE_MAPPING is defined. This causes zsmalloc to use page table
> >- * mapping rather than copying
> >- * for object mapping.
> >+ * performs VM mapping faster than copying, then it should be added here
> >+ * so that USE_PGTABLE_MAPPING is defined. This causes zsmalloc to use
> >+ * page table mapping rather than copying for object mapping.
> >  */
> >  #if defined(CONFIG_ARM)
> >  #define USE_PGTABLE_MAPPING
> >@@ -663,7 +661,7 @@ static inline void __zs_unmap_object(struct mapping_area *area,
> >  	flush_cache_vunmap(addr, end);
> >  	unmap_kernel_range_noflush(addr, PAGE_SIZE * 2);
> >-	local_flush_tlb_kernel_range(addr, end);
> >+	flush_tlb_kernel_range(addr, end);
> >  }
> >  #else /* USE_PGTABLE_MAPPING */
> 
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-- 
Kind regards,
Minchan Kim

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