Commit ce474796 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'x86-mm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 

Pull x86 mm changes from Ingo Molnar:
 "The main change in this cycle is the rework of the TLB range flushing
  code, to simplify, fix and consolidate the code.  By Dave Hansen"

* 'x86-mm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/mm: Set TLB flush tunable to sane value (33)
  x86/mm: New tunable for single vs full TLB flush
  x86/mm: Add tracepoints for TLB flushes
  x86/mm: Unify remote INVLPG code
  x86/mm: Fix missed global TLB flush stat
  x86/mm: Rip out complicated, out-of-date, buggy TLB flushing
  x86/mm: Clean up the TLB flushing code
  x86/smep: Be more informative when signalling an SMEP fault
parents 76f09aa4 a5102476

Documentation/x86/tlb.txt

0 → 100644
+75 −0
Original line number Diff line number Diff line 
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 "4.10.4.2 Recommended Invalidation"

   says: "One execution of INVLPG is sufficient even for a page

   with size greater than 4 KBytes."

arch/x86/include/asm/mmu_context.h

+6 −0
Original line number Diff line number Diff line
@@ -3,6 +3,10 @@ 


#include <asm/desc.h>

#include <linux/atomic.h>

#include <linux/mm_types.h>



#include <trace/events/tlb.h>



#include <asm/pgalloc.h>

#include <asm/tlbflush.h>

#include <asm/paravirt.h>
@@ -44,6 +48,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, 


		/* Re-load page tables */

		load_cr3(next->pgd);

		trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);



		/* Stop flush ipis for the previous mm */

		cpumask_clear_cpu(cpu, mm_cpumask(prev));
@@ -71,6 +76,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, 
			 * to make sure to use no freed page tables.

			 */

			load_cr3(next->pgd);

			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);

			load_LDT_nolock(&next->context);

		}

	}

arch/x86/include/asm/processor.h

+0 −1
Original line number Diff line number Diff line
@@ -72,7 +72,6 @@ extern u16 __read_mostly tlb_lld_4k[NR_INFO]; 
extern u16 __read_mostly tlb_lld_2m[NR_INFO];

extern u16 __read_mostly tlb_lld_4m[NR_INFO];

extern u16 __read_mostly tlb_lld_1g[NR_INFO];

extern s8  __read_mostly tlb_flushall_shift;



/*

 *  CPU type and hardware bug flags. Kept separately for each CPU.

arch/x86/kernel/cpu/amd.c

+0 −7
Original line number Diff line number Diff line
@@ -724,11 +724,6 @@ static unsigned int amd_size_cache(struct cpuinfo_x86 *c, unsigned int size) 
}

#endif



static void cpu_set_tlb_flushall_shift(struct cpuinfo_x86 *c)

{

	tlb_flushall_shift = 6;

}



static void cpu_detect_tlb_amd(struct cpuinfo_x86 *c)

{

	u32 ebx, eax, ecx, edx;
@@ -776,8 +771,6 @@ static void cpu_detect_tlb_amd(struct cpuinfo_x86 *c) 
		tlb_lli_2m[ENTRIES] = eax & mask;



	tlb_lli_4m[ENTRIES] = tlb_lli_2m[ENTRIES] >> 1;



	cpu_set_tlb_flushall_shift(c);

}



static const struct cpu_dev amd_cpu_dev = {

arch/x86/kernel/cpu/common.c

+2 −11
Original line number Diff line number Diff line
@@ -481,26 +481,17 @@ u16 __read_mostly tlb_lld_2m[NR_INFO]; 
u16 __read_mostly tlb_lld_4m[NR_INFO];

u16 __read_mostly tlb_lld_1g[NR_INFO];



/*

 * tlb_flushall_shift shows the balance point in replacing cr3 write

 * with multiple 'invlpg'. It will do this replacement when

 *   flush_tlb_lines <= active_lines/2^tlb_flushall_shift.

 * If tlb_flushall_shift is -1, means the replacement will be disabled.

 */

s8  __read_mostly tlb_flushall_shift = -1;



void cpu_detect_tlb(struct cpuinfo_x86 *c)

{

	if (this_cpu->c_detect_tlb)

		this_cpu->c_detect_tlb(c);



	printk(KERN_INFO "Last level iTLB entries: 4KB %d, 2MB %d, 4MB %d\n"

		"Last level dTLB entries: 4KB %d, 2MB %d, 4MB %d, 1GB %d\n"

		"tlb_flushall_shift: %d\n",

		"Last level dTLB entries: 4KB %d, 2MB %d, 4MB %d, 1GB %d\n",

		tlb_lli_4k[ENTRIES], tlb_lli_2m[ENTRIES],

		tlb_lli_4m[ENTRIES], tlb_lld_4k[ENTRIES],

		tlb_lld_2m[ENTRIES], tlb_lld_4m[ENTRIES],

		tlb_lld_1g[ENTRIES], tlb_flushall_shift);

		tlb_lld_1g[ENTRIES]);

}



void detect_ht(struct cpuinfo_x86 *c)

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