linux-kernel - Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

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Message-ID: <24277842-c920-4a12-57d1-2ebcdf3c1534@intel.com>
Date:   Mon, 4 Feb 2019 11:44:29 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, alexander.h.duyck@...ux.intel.com,
        x86@...nel.org, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        pbonzini@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

On 2/4/19 10:15 AM, Alexander Duyck wrote:
> +#ifdef CONFIG_KVM_GUEST
> +#include <linux/jump_label.h>
> +extern struct static_key_false pv_free_page_hint_enabled;
> +
> +#define HAVE_ARCH_FREE_PAGE
> +void __arch_free_page(struct page *page, unsigned int order);
> +static inline void arch_free_page(struct page *page, unsigned int order)
> +{
> +	if (static_branch_unlikely(&pv_free_page_hint_enabled))
> +		__arch_free_page(page, order);
> +}
> +#endif

So, this ends up with at least a call, a branch and a ret added to the
order-0 paths, including freeing pages to the per-cpu-pageset lists.
That seems worrisome.

What performance testing has been performed to look into the overhead
added to those paths?