linux-kernel - Re: [PATCH 01/26] x86/sgx: Call cond_resched() at the end of sgx_reclaim

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Date:   Wed, 16 Nov 2022 01:27:35 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     dave.hansen@...ux.kernel.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        cgroups@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        zhiquan1.li@...el.com, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 01/26] x86/sgx: Call cond_resched() at the end of
 sgx_reclaim_pages()

On Fri, Nov 11, 2022 at 10:35:06AM -0800, Kristen Carlson Accardi wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> In order to avoid repetition of cond_resched() in ksgxd() and
> sgx_alloc_epc_page(), move the invocation of post-reclaim cond_resched()
> inside sgx_reclaim_pages(). Except in the case of sgx_reclaim_direct(),
> sgx_reclaim_pages() is always called in a loop and is always followed
> by a call to cond_resched().  This will hold true for the EPC cgroup
> as well, which adds even more calls to sgx_reclaim_pages() and thus
> cond_resched(). Calls to sgx_reclaim_direct() may be performance
> sensitive. Allow sgx_reclaim_direct() to avoid the cond_resched()
> call by moving the original sgx_reclaim_pages() call to
> __sgx_reclaim_pages() and then have sgx_reclaim_pages() become a
> wrapper around that call with a cond_resched().
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 160c8dbee0ab..ffce6fc70a1f 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -287,7 +287,7 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,
>   * problematic as it would increase the lock contention too much, which would
>   * halt forward progress.
>   */
> -static void sgx_reclaim_pages(void)
> +static void __sgx_reclaim_pages(void)
>  {
>  	struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
>  	struct sgx_backing backing[SGX_NR_TO_SCAN];
> @@ -369,6 +369,12 @@ static void sgx_reclaim_pages(void)
>  	}
>  }
>  
> +static void sgx_reclaim_pages(void)
> +{
> +	__sgx_reclaim_pages();
> +	cond_resched();
> +}
> +
>  static bool sgx_should_reclaim(unsigned long watermark)
>  {
>  	return atomic_long_read(&sgx_nr_free_pages) < watermark &&
> @@ -378,12 +384,14 @@ static bool sgx_should_reclaim(unsigned long watermark)
>  /*
>   * sgx_reclaim_direct() should be called (without enclave's mutex held)
>   * in locations where SGX memory resources might be low and might be
> - * needed in order to make forward progress.
> + * needed in order to make forward progress. This call to
> + * __sgx_reclaim_pages() avoids the cond_resched() in sgx_reclaim_pages()
> + * to improve performance.
>   */
>  void sgx_reclaim_direct(void)
>  {
>  	if (sgx_should_reclaim(SGX_NR_LOW_PAGES))
> -		sgx_reclaim_pages();
> +		__sgx_reclaim_pages();

Is it a big deal to have "extra" cond_resched?

>  }
>  
>  static int ksgxd(void *p)
> @@ -410,8 +418,6 @@ static int ksgxd(void *p)
>  
>  		if (sgx_should_reclaim(SGX_NR_HIGH_PAGES))
>  			sgx_reclaim_pages();
> -
> -		cond_resched();
>  	}
>  
>  	return 0;
> @@ -582,7 +588,6 @@ struct sgx_epc_page *sgx_alloc_epc_page(void *owner, bool reclaim)
>  		}
>  
>  		sgx_reclaim_pages();
> -		cond_resched();
>  	}
>  
>  	if (sgx_should_reclaim(SGX_NR_LOW_PAGES))
> -- 
> 2.37.3
> 

BR, Jarkko