Message-ID: <1545B936-7CEC-4A1C-B776-74004F774218@vmware.com>
Date:   Wed, 26 Jun 2019 02:35:00 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 4/9] x86/mm/tlb: Flush remote and local TLBs concurrently

> On Jun 25, 2019, at 2:29 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 6/12/19 11:48 PM, Nadav Amit wrote:
>> To improve TLB shootdown performance, flush the remote and local TLBs
>> concurrently. Introduce flush_tlb_multi() that does so. The current
>> flush_tlb_others() interface is kept, since paravirtual interfaces need
>> to be adapted first before it can be removed. This is left for future
>> work. In such PV environments, TLB flushes are not performed, at this
>> time, concurrently.
>> 
>> Add a static key to tell whether this new interface is supported.
>> 
>> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
>> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
>> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
>> Cc: Sasha Levin <sashal@...nel.org>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Borislav Petkov <bp@...en8.de>
>> Cc: x86@...nel.org
>> Cc: Juergen Gross <jgross@...e.com>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Andy Lutomirski <luto@...nel.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>> Cc: linux-hyperv@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: virtualization@...ts.linux-foundation.org
>> Cc: kvm@...r.kernel.org
>> Cc: xen-devel@...ts.xenproject.org
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> arch/x86/hyperv/mmu.c                 |  2 +
>> arch/x86/include/asm/paravirt.h       |  8 +++
>> arch/x86/include/asm/paravirt_types.h |  6 +++
>> arch/x86/include/asm/tlbflush.h       |  6 +++
>> arch/x86/kernel/kvm.c                 |  1 +
>> arch/x86/kernel/paravirt.c            |  3 ++
>> arch/x86/mm/tlb.c                     | 71 ++++++++++++++++++++++-----
>> arch/x86/xen/mmu_pv.c                 |  2 +
>> 8 files changed, 87 insertions(+), 12 deletions(-)
>> 
>> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
>> index e65d7fe6489f..ca28b400c87c 100644
>> --- a/arch/x86/hyperv/mmu.c
>> +++ b/arch/x86/hyperv/mmu.c
>> @@ -233,4 +233,6 @@ void hyperv_setup_mmu_ops(void)
>> 	pr_info("Using hypercall for remote TLB flush\n");
>> 	pv_ops.mmu.flush_tlb_others = hyperv_flush_tlb_others;
>> 	pv_ops.mmu.tlb_remove_table = tlb_remove_table;
>> +
>> +	static_key_disable(&flush_tlb_multi_enabled.key);
>> }
>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>> index c25c38a05c1c..192be7254457 100644
>> --- a/arch/x86/include/asm/paravirt.h
>> +++ b/arch/x86/include/asm/paravirt.h
>> @@ -47,6 +47,8 @@ static inline void slow_down_io(void)
>> #endif
>> }
>> 
>> +DECLARE_STATIC_KEY_TRUE(flush_tlb_multi_enabled);
>> +
>> static inline void __flush_tlb(void)
>> {
>> 	PVOP_VCALL0(mmu.flush_tlb_user);
>> @@ -62,6 +64,12 @@ static inline void __flush_tlb_one_user(unsigned long addr)
>> 	PVOP_VCALL1(mmu.flush_tlb_one_user, addr);
>> }
>> 
>> +static inline void flush_tlb_multi(const struct cpumask *cpumask,
>> +				   const struct flush_tlb_info *info)
>> +{
>> +	PVOP_VCALL2(mmu.flush_tlb_multi, cpumask, info);
>> +}
>> +
>> static inline void flush_tlb_others(const struct cpumask *cpumask,
>> 				    const struct flush_tlb_info *info)
>> {
>> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
>> index 946f8f1f1efc..b93b3d90729a 100644
>> --- a/arch/x86/include/asm/paravirt_types.h
>> +++ b/arch/x86/include/asm/paravirt_types.h
>> @@ -211,6 +211,12 @@ struct pv_mmu_ops {
>> 	void (*flush_tlb_user)(void);
>> 	void (*flush_tlb_kernel)(void);
>> 	void (*flush_tlb_one_user)(unsigned long addr);
>> +	/*
>> +	 * flush_tlb_multi() is the preferred interface, which is capable to
>> +	 * flush both local and remote CPUs.
>> +	 */
>> +	void (*flush_tlb_multi)(const struct cpumask *cpus,
>> +				const struct flush_tlb_info *info);
>> 	void (*flush_tlb_others)(const struct cpumask *cpus,
>> 				 const struct flush_tlb_info *info);
>> 
>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>> index dee375831962..79272938cf79 100644
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -569,6 +569,9 @@ static inline void flush_tlb_page(struct vm_area_struct *vma, unsigned long a)
>> 	flush_tlb_mm_range(vma->vm_mm, a, a + PAGE_SIZE, PAGE_SHIFT, false);
>> }
>> 
>> +void native_flush_tlb_multi(const struct cpumask *cpumask,
>> +			     const struct flush_tlb_info *info);
>> +
>> void native_flush_tlb_others(const struct cpumask *cpumask,
>> 			     const struct flush_tlb_info *info);
>> 
>> @@ -593,6 +596,9 @@ static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
>> extern void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
>> 
>> #ifndef CONFIG_PARAVIRT
>> +#define flush_tlb_multi(mask, info)	\
>> +	native_flush_tlb_multi(mask, info)
>> +
>> #define flush_tlb_others(mask, info)	\
>> 	native_flush_tlb_others(mask, info)
>> 
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 5169b8cc35bb..00d81e898717 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -630,6 +630,7 @@ static void __init kvm_guest_init(void)
>> 	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
>> 		pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
>> 		pv_ops.mmu.tlb_remove_table = tlb_remove_table;
>> +		static_key_disable(&flush_tlb_multi_enabled.key);
>> 	}
>> 
>> 	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
>> index 98039d7fb998..ac00afed5570 100644
>> --- a/arch/x86/kernel/paravirt.c
>> +++ b/arch/x86/kernel/paravirt.c
>> @@ -159,6 +159,8 @@ unsigned paravirt_patch_insns(void *insn_buff, unsigned len,
>> 	return insn_len;
>> }
>> 
>> +DEFINE_STATIC_KEY_TRUE(flush_tlb_multi_enabled);
>> +
>> static void native_flush_tlb(void)
>> {
>> 	__native_flush_tlb();
>> @@ -363,6 +365,7 @@ struct paravirt_patch_template pv_ops = {
>> 	.mmu.flush_tlb_user	= native_flush_tlb,
>> 	.mmu.flush_tlb_kernel	= native_flush_tlb_global,
>> 	.mmu.flush_tlb_one_user	= native_flush_tlb_one_user,
>> +	.mmu.flush_tlb_multi	= native_flush_tlb_multi,
>> 	.mmu.flush_tlb_others	= native_flush_tlb_others,
>> 	.mmu.tlb_remove_table	=
>> 			(void (*)(struct mmu_gather *, void *))tlb_remove_page,
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index c34bcf03f06f..db73d5f1dd43 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -551,7 +551,7 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
>> 		 * garbage into our TLB.  Since switching to init_mm is barely
>> 		 * slower than a minimal flush, just switch to init_mm.
>> 		 *
>> -		 * This should be rare, with native_flush_tlb_others skipping
>> +		 * This should be rare, with native_flush_tlb_multi skipping
>> 		 * IPIs to lazy TLB mode CPUs.
>> 		 */
> 
> Nit, since we're messing with this, it can now be
> "native_flush_tlb_multi()" since it is a function.

Sure.

> 
>> switch_mm_irqs_off(NULL, &init_mm, NULL);
>> @@ -635,9 +635,12 @@ 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(const void *info, enum tlb_flush_reason reason)
>> +static void flush_tlb_func_local(void *info)
>> {
>> 	const struct flush_tlb_info *f = info;
>> +	enum tlb_flush_reason reason;
>> +
>> +	reason = (f->mm == NULL) ? TLB_LOCAL_SHOOTDOWN : TLB_LOCAL_MM_SHOOTDOWN;
> 
> Should we just add the "reason" to flush_tlb_info?  It's OK-ish to imply
> it like this, but seems like it would be nicer and easier to track down
> the origins of these things if we did this at the caller.

I prefer not to. I want later to inline flush_tlb_info into the same
cacheline that holds call_function_data. Increasing the size of
flush_tlb_info for no good reason will not help…

>> flush_tlb_func_common(f, true, reason);
>> }
>> @@ -655,14 +658,21 @@ static void flush_tlb_func_remote(void *info)
>> 	flush_tlb_func_common(f, false, TLB_REMOTE_SHOOTDOWN);
>> }
>> 
>> -static bool tlb_is_not_lazy(int cpu, void *data)
>> +static inline bool tlb_is_not_lazy(int cpu)
>> {
>> 	return !per_cpu(cpu_tlbstate.is_lazy, cpu);
>> }
> 
> Nit: the compiler will probably inline this sucker anyway.  So, for
> these kinds of patches, I'd resist the urge to do these kinds of tweaks,
> especially since it starts to hide the important change on the line.

Of course.

> 
>> -void native_flush_tlb_others(const struct cpumask *cpumask,
>> -			     const struct flush_tlb_info *info)
>> +static DEFINE_PER_CPU(cpumask_t, flush_tlb_mask);
>> +
>> +void native_flush_tlb_multi(const struct cpumask *cpumask,
>> +			    const struct flush_tlb_info *info)
>> {
>> +	/*
>> +	 * Do accounting and tracing. Note that there are (and have always been)
>> +	 * cases in which a remote TLB flush will be traced, but eventually
>> +	 * would not happen.
>> +	 */
>> 	count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
>> 	if (info->end == TLB_FLUSH_ALL)
>> 		trace_tlb_flush(TLB_REMOTE_SEND_IPI, TLB_FLUSH_ALL);
>> @@ -682,10 +692,14 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
>> 		 * means that the percpu tlb_gen variables won't be updated
>> 		 * and we'll do pointless flushes on future context switches.
>> 		 *
>> -		 * Rather than hooking native_flush_tlb_others() here, I think
>> +		 * Rather than hooking native_flush_tlb_multi() here, I think
>> 		 * that UV should be updated so that smp_call_function_many(),
>> 		 * etc, are optimal on UV.
>> 		 */
>> +		local_irq_disable();
>> +		flush_tlb_func_local((__force void *)info);
>> +		local_irq_enable();
>> +
>> 		cpumask = uv_flush_tlb_others(cpumask, info);
>> 		if (cpumask)
>> 			smp_call_function_many(cpumask, flush_tlb_func_remote,
>> @@ -704,11 +718,39 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
>> 	 * doing a speculative memory access.
>> 	 */
>> 	if (info->freed_tables)
>> -		smp_call_function_many(cpumask, flush_tlb_func_remote,
>> -			       (void *)info, 1);
>> -	else
>> -		on_each_cpu_cond_mask(tlb_is_not_lazy, flush_tlb_func_remote,
>> -				(void *)info, 1, GFP_ATOMIC, cpumask);
>> +		__smp_call_function_many(cpumask, flush_tlb_func_remote,
>> +					 flush_tlb_func_local, (void *)info, 1);
>> +	else {
> 
> I prefer brackets be added for 'if' blocks like this since it doesn't
> take up any meaningful space and makes it less prone to compile errors.

If you say so.

> 
>> +		/*
>> +		 * Although we could have used on_each_cpu_cond_mask(),
>> +		 * open-coding it has several performance advantages: (1) we can
>> +		 * use specialized functions for remote and local flushes; (2)
>> +		 * no need for indirect branch to test if TLB is lazy; (3) we
>> +		 * can use a designated cpumask for evaluating the condition
>> +		 * instead of allocating a new one.
>> +		 *
>> +		 * This works under the assumption that there are no nested TLB
>> +		 * flushes, an assumption that is already made in
>> +		 * flush_tlb_mm_range().
>> +		 */
>> +		struct cpumask *cond_cpumask = this_cpu_ptr(&flush_tlb_mask);
> 
> This is logically a stack-local variable, right?  But, since we've got
> preempt off and cpumasks can be huge, we don't want to allocate it on
> the stack.  That might be worth a comment somewhere.

I will add a comment here.

> 
>> +		int cpu;
>> +
>> +		cpumask_clear(cond_cpumask);
>> +
>> +		for_each_cpu(cpu, cpumask) {
>> +			if (tlb_is_not_lazy(cpu))
>> +				__cpumask_set_cpu(cpu, cond_cpumask);
>> +		}
> 
> FWIW, it's probably worth calling out in the changelog that this loop
> exists in on_each_cpu_cond_mask() too.  It looks bad here, but it's no
> worse than what it replaces.

Added.

> 
>> +		__smp_call_function_many(cond_cpumask, flush_tlb_func_remote,
>> +					 flush_tlb_func_local, (void *)info, 1);
>> +	}
>> +}
> 
> There was a __force on an earlier 'info' cast.  Could you talk about
> that for a minute an explain why that one is needed?

I have no idea where the __force came from. I’ll remove it.

> 
>> +void native_flush_tlb_others(const struct cpumask *cpumask,
>> +			     const struct flush_tlb_info *info)
>> +{
>> +	native_flush_tlb_multi(cpumask, info);
>> }
>> 
>> /*
>> @@ -774,10 +816,15 @@ static void flush_tlb_on_cpus(const cpumask_t *cpumask,
>> {
>> 	int this_cpu = smp_processor_id();
>> 
>> +	if (static_branch_likely(&flush_tlb_multi_enabled)) {
>> +		flush_tlb_multi(cpumask, info);
>> +		return;
>> +	}
> 
> Probably needs a comment for posterity above the if()^^:
> 
> 	/* Use the optimized flush_tlb_multi() where we can. */

Right.

> 
>> --- a/arch/x86/xen/mmu_pv.c
>> +++ b/arch/x86/xen/mmu_pv.c
>> @@ -2474,6 +2474,8 @@ void __init xen_init_mmu_ops(void)
>> 
>> 	pv_ops.mmu = xen_mmu_ops;
>> 
>> +	static_key_disable(&flush_tlb_multi_enabled.key);
>> +
>> 	memset(dummy_mapping, 0xff, PAGE_SIZE);
>> }
> 
> More comments, please.  Perhaps:
> 
> 	Existing paravirt TLB flushes are incompatible with
> 	flush_tlb_multi() because....  Disable it when they are
> 	in use.

There is no inherent reason for them to be incompatible. Someone needs to
adapt them. I will use my affiliation as an excuse for the question “why
don’t you do it?” ;-)

Anyhow, I will add a comment.

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