RE: [PATCH v2 2/2] x86/hyperv: remove on-stack cpumask from hv_send_ipi_mask_allbutself

[Michael Kelley]

From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Friday, September 10, 2021 11:57 AM
> 
> It is not a good practice to allocate a cpumask on stack, given it may
> consume up to 1 kilobytes of stack space if the kernel is configured to
> have 8192 cpus.
> 
> The internal helper functions __send_ipi_mask{,_ex} need to loop over
> the provided mask anyway, so it is not too difficult to skip `self'
> there. We can thus do away with the on-stack cpumask in
> hv_send_ipi_mask_allbutself.
> 
> Adjust call sites of __send_ipi_mask as needed.
> 
> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Fixes: 68bb7bfb7985d ("X86/Hyper-V: Enable IPI enlightenments")
> Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> ---
> 
> v2: more robust check in __send_ipi_mask
> ---
>  arch/x86/hyperv/hv_apic.c | 43 +++++++++++++++++++++++----------------
>  1 file changed, 26 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
> index 90e682a92820..48aefcea724b 100644
> --- a/arch/x86/hyperv/hv_apic.c
> +++ b/arch/x86/hyperv/hv_apic.c
> @@ -99,7 +99,8 @@ static void hv_apic_eoi_write(u32 reg, u32 val)
>  /*
>   * IPI implementation on Hyper-V.
>   */
> -static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector)
> +static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector,
> +		bool exclude_self)
>  {
>  	struct hv_send_ipi_ex **arg;
>  	struct hv_send_ipi_ex *ipi_arg;
> @@ -123,7 +124,10 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector)
> 
>  	if (!cpumask_equal(mask, cpu_present_mask)) {
>  		ipi_arg->vp_set.format = HV_GENERIC_SET_SPARSE_4K;
> -		nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
> +		if (exclude_self)
> +			nr_bank = cpumask_to_vpset_noself(&(ipi_arg->vp_set), mask);
> +		else
> +			nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
>  	}
>  	if (nr_bank < 0)
>  		goto ipi_mask_ex_done;
> @@ -138,15 +142,25 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector)
>  	return hv_result_success(status);
>  }
> 
> -static bool __send_ipi_mask(const struct cpumask *mask, int vector)
> +static bool __send_ipi_mask(const struct cpumask *mask, int vector,
> +		bool exclude_self)
>  {
> -	int cur_cpu, vcpu;
> +	int cur_cpu, vcpu, this_cpu = smp_processor_id();
>  	struct hv_send_ipi ipi_arg;
>  	u64 status;
> +	unsigned int weight;
> 
>  	trace_hyperv_send_ipi_mask(mask, vector);
> 
> -	if (cpumask_empty(mask))
> +	weight = cpumask_weight(mask);
> +
> +	/*
> +	 * Do nothing if
> +	 *   1. the mask is empty
> +	 *   2. the mask only contains self when exclude_self is true
> +	 */
> +	if (weight == 0 ||
> +	    (exclude_self && weight == 1 && cpumask_first(mask) == this_cpu))

Nit:  cpumask_test_cpu(this_cpu, mask) would seem to be a better fit for this
use case than cpumask_first().  But either works.

>  		return true;
> 
>  	if (!hv_hypercall_pg)
> @@ -172,6 +186,8 @@ static bool __send_ipi_mask(const struct cpumask *mask, int vector)
>  	ipi_arg.cpu_mask = 0;
> 
>  	for_each_cpu(cur_cpu, mask) {
> +		if (exclude_self && cur_cpu == this_cpu)
> +			continue;
>  		vcpu = hv_cpu_number_to_vp_number(cur_cpu);
>  		if (vcpu == VP_INVAL)
>  			return false;
> @@ -191,7 +207,7 @@ static bool __send_ipi_mask(const struct cpumask *mask, int vector)
>  	return hv_result_success(status);
> 
>  do_ex_hypercall:
> -	return __send_ipi_mask_ex(mask, vector);
> +	return __send_ipi_mask_ex(mask, vector, exclude_self);
>  }
> 
>  static bool __send_ipi_one(int cpu, int vector)
> @@ -208,7 +224,7 @@ static bool __send_ipi_one(int cpu, int vector)
>  		return false;
> 
>  	if (vp >= 64)
> -		return __send_ipi_mask_ex(cpumask_of(cpu), vector);
> +		return __send_ipi_mask_ex(cpumask_of(cpu), vector, false);
> 
>  	status = hv_do_fast_hypercall16(HVCALL_SEND_IPI, vector, BIT_ULL(vp));
>  	return hv_result_success(status);
> @@ -222,20 +238,13 @@ static void hv_send_ipi(int cpu, int vector)
> 
>  static void hv_send_ipi_mask(const struct cpumask *mask, int vector)
>  {
> -	if (!__send_ipi_mask(mask, vector))
> +	if (!__send_ipi_mask(mask, vector, false))
>  		orig_apic.send_IPI_mask(mask, vector);
>  }
> 
>  static void hv_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  {
> -	unsigned int this_cpu = smp_processor_id();
> -	struct cpumask new_mask;
> -	const struct cpumask *local_mask;
> -
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> -	if (!__send_ipi_mask(local_mask, vector))
> +	if (!__send_ipi_mask(mask, vector, true))
>  		orig_apic.send_IPI_mask_allbutself(mask, vector);
>  }
> 
> @@ -246,7 +255,7 @@ static void hv_send_ipi_allbutself(int vector)
> 
>  static void hv_send_ipi_all(int vector)
>  {
> -	if (!__send_ipi_mask(cpu_online_mask, vector))
> +	if (!__send_ipi_mask(cpu_online_mask, vector, false))
>  		orig_apic.send_IPI_all(vector);
>  }
> 
> --
> 2.30.2

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>