linux-kernel - Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb

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Date:   Thu, 25 Apr 2019 21:20:24 +0000
From:   Nadav Amit <namit@...are.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb_info from the stack

> On Apr 25, 2019, at 12:48 PM, Ingo Molnar <mingo@...nel.org> wrote:
> 
> 
> * Nadav Amit <namit@...are.com> wrote:
> 
>>> On Apr 25, 2019, at 12:29 PM, Ingo Molnar <mingo@...nel.org> wrote:
>>> 
>>> 
>>> * Nadav Amit <namit@...are.com> wrote:
>>> 
>>>> Move flush_tlb_info variables off the stack. This allows to align
>>>> flush_tlb_info to cache-line and avoid potentially unnecessary cache
>>>> line movements. It also allows to have a fixed virtual-to-physical
>>>> translation of the variables, which reduces TLB misses.
>>>> 
>>>> Use per-CPU struct for flush_tlb_mm_range() and
>>>> flush_tlb_kernel_range(). Add debug assertions to ensure there are
>>>> no nested TLB flushes that might overwrite the per-CPU data. For
>>>> arch_tlbbatch_flush() use a const struct.
>>>> 
>>>> Results when running a microbenchmarks that performs 10^6 MADV_DONTEED
>>>> operations and touching a page, in which 3 additional threads run a
>>>> busy-wait loop (5 runs, PTI and retpolines are turned off):
>>>> 
>>>> 			base		off-stack
>>>> 			----		---------
>>>> avg (usec/op)		1.629		1.570	(-3%)
>>>> stddev			0.014		0.009
>>>> 
>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>> Cc: Andy Lutomirski <luto@...nel.org>
>>>> Cc: Dave Hansen <dave.hansen@...el.com>
>>>> Cc: Borislav Petkov <bp@...en8.de>
>>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>>> Signed-off-by: Nadav Amit <namit@...are.com>
>>>> 
>>>> ---
>>>> 
>>>> v1->v2:
>>>> - Initialize all flush_tlb_info fields [Andy]
>>>> ---
>>>> arch/x86/mm/tlb.c | 100 ++++++++++++++++++++++++++++++++++------------
>>>> 1 file changed, 74 insertions(+), 26 deletions(-)
>>>> 
>>>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>>>> index 487b8474c01c..aac191eb2b90 100644
>>>> --- a/arch/x86/mm/tlb.c
>>>> +++ b/arch/x86/mm/tlb.c
>>>> @@ -634,7 +634,7 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
>>>> 	this_cpu_write(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen, mm_tlb_gen);
>>>> }
>>>> 
>>>> -static void flush_tlb_func_local(void *info, enum tlb_flush_reason reason)
>>>> +static void flush_tlb_func_local(const void *info, enum tlb_flush_reason reason)
>>>> {
>>>> 	const struct flush_tlb_info *f = info;
>>>> 
>>>> @@ -722,43 +722,81 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
>>>> */
>>>> unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;
>>>> 
>>>> +static DEFINE_PER_CPU_SHARED_ALIGNED(struct flush_tlb_info, flush_tlb_info);
>>>> +
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> +static DEFINE_PER_CPU(unsigned int, flush_tlb_info_idx);
>>>> +#endif
>>>> +
>>>> +static inline struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
>>>> +			unsigned long start, unsigned long end,
>>>> +			unsigned int stride_shift, bool freed_tables,
>>>> +			u64 new_tlb_gen)
>>>> +{
>>>> +	struct flush_tlb_info *info = this_cpu_ptr(&flush_tlb_info);
>>>> +
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> +	/*
>>>> +	 * Ensure that the following code is non-reentrant and flush_tlb_info
>>>> +	 * is not overwritten. This means no TLB flushing is initiated by
>>>> +	 * interrupt handlers and machine-check exception handlers.
>>>> +	 */
>>>> +	BUG_ON(this_cpu_inc_return(flush_tlb_info_idx) != 1);
>>>> +#endif
>>> 
>>> isn't this effectively VM_BUG_ON()?
>> 
>> Not exactly. When CONFIG_DEBUG_VM is off we get
>> 
>> 	#define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
>> 
>> This will cause the build to fail since flush_tlb_info_idx is not defined in
>> when CONFIG_DEBUG_VM is off.
> 
> Ugh, so VM_BUG_ON() should really be named VM_BUILD_BUG_ON()?
> 
> Anyway, agreed.
> 
>>>> +static inline void put_flush_tlb_info(void)
>>>> +{
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> +	/* Complete reentrency prevention checks */
>>>> +	barrier();
>>>> +	this_cpu_dec(flush_tlb_info_idx);
>>>> +#endif
>>> 
>>> In principle this_cpu_dec() should imply a compiler barrier?
>> 
>> this_cpu_dec() is eventually expanded to the macro of percpu_add_op(). And
>> the inline assembly does not have a “memory” clobber, so I don’t think so.
> 
> I think that's a bug and PeterZ is fixing those.

This would be quite surprising. Even atomic_dec() does not imply a compilers
barrier. I think I should leave it as is for now, and let Peter change it
later if he decides to do so. Let me know if you disagree.