path: root/Documentation/x86/tlb.txt
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authorDave Hansen <dave.hansen@linux.intel.com>2014-07-31 08:41:01 -0700
committerH. Peter Anvin <hpa@linux.intel.com>2014-07-31 08:48:51 -0700
commit2d040a1ce903ca5d6e7c983621fb29c6883c4c48 (patch)
tree2969f9de20422d8e247cf6e13d043330878baff6 /Documentation/x86/tlb.txt
parentd17d8f9dedb9dd76fd540a5c497101529d9eb25a (diff)
x86/mm: New tunable for single vs full TLB flush
Most of the logic here is in the documentation file. Please take a look at it. I know we've come full-circle here back to a tunable, but this new one is *WAY* simpler. I challenge anyone to describe in one sentence how the old one worked. Here's the way the new one works: If we are flushing more pages than the ceiling, we use the full flush, otherwise we use per-page flushes. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Link: http://lkml.kernel.org/r/20140731154101.12B52CAF@viggo.jf.intel.com Acked-by: Rik van Riel <riel@redhat.com> Acked-by: Mel Gorman <mgorman@suse.de> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
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+When the kernel unmaps or modified the attributes of a range of
+memory, it has two choices:
+ 1. Flush the entire TLB with a two-instruction sequence. This is
+ a quick operation, but it causes collateral damage: TLB entries
+ from areas other than the one we are trying to flush will be
+ destroyed and must be refilled later, at some cost.
+ 2. Use the invlpg instruction to invalidate a single page at a
+ time. This could potentialy cost many more instructions, but
+ it is a much more precise operation, causing no collateral
+ damage to other TLB entries.
+Which method to do depends on a few things:
+ 1. The size of the flush being performed. A flush of the entire
+ address space is obviously better performed by flushing the
+ entire TLB than doing 2^48/PAGE_SIZE individual flushes.
+ 2. The contents of the TLB. If the TLB is empty, then there will
+ be no collateral damage caused by doing the global flush, and
+ all of the individual flush will have ended up being wasted
+ work.
+ 3. The size of the TLB. The larger the TLB, the more collateral
+ damage we do with a full flush. So, the larger the TLB, the
+ more attrative an individual flush looks. Data and
+ instructions have separate TLBs, as do different page sizes.
+ 4. The microarchitecture. The TLB has become a multi-level
+ cache on modern CPUs, and the global flushes have become more
+ expensive relative to single-page flushes.
+There is obviously no way the kernel can know all these things,
+especially the contents of the TLB during a given flush. The
+sizes of the flush will vary greatly depending on the workload as
+well. There is essentially no "right" point to choose.
+You may be doing too many individual invalidations if you see the
+invlpg instruction (or instructions _near_ it) show up high in
+profiles. If you believe that individual invalidations being
+called too often, you can lower the tunable:
+ /sys/debug/kernel/x86/tlb_single_page_flush_ceiling
+This will cause us to do the global flush for more cases.
+Lowering it to 0 will disable the use of the individual flushes.
+Setting it to 1 is a very conservative setting and it should
+never need to be 0 under normal circumstances.
+Despite the fact that a single individual flush on x86 is
+guaranteed to flush a full 2MB [1], hugetlbfs always uses the full
+flushes. THP is treated exactly the same as normal memory.
+You might see invlpg inside of flush_tlb_mm_range() show up in
+profiles, or you can use the trace_tlb_flush() tracepoints. to
+determine how long the flush operations are taking.
+Essentially, you are balancing the cycles you spend doing invlpg
+with the cycles that you spend refilling the TLB later.
+You can measure how expensive TLB refills are by using
+performance counters and 'perf stat', like this:
+perf stat -e
+ cpu/event=0x8,umask=0x84,name=dtlb_load_misses_walk_duration/,
+ cpu/event=0x8,umask=0x82,name=dtlb_load_misses_walk_completed/,
+ cpu/event=0x49,umask=0x4,name=dtlb_store_misses_walk_duration/,
+ cpu/event=0x49,umask=0x2,name=dtlb_store_misses_walk_completed/,
+ cpu/event=0x85,umask=0x4,name=itlb_misses_walk_duration/,
+ cpu/event=0x85,umask=0x2,name=itlb_misses_walk_completed/
+That works on an IvyBridge-era CPU (i5-3320M). Different CPUs
+may have differently-named counters, but they should at least
+be there in some form. You can use pmu-tools 'ocperf list'
+(https://github.com/andikleen/pmu-tools) to find the right
+counters for a given CPU.
+1. A footnote in Intel's SDM " Recommended Invalidation"
+ says: "One execution of INVLPG is sufficient even for a page
+ with size greater than 4 KBytes."

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