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X-CSE-ConnectionGUID: 1zSFvLTcRu+mNv2WDekngg== X-CSE-MsgGUID: TtSCZN62Sdih4wicYDLOgQ== X-IronPort-AV: E=McAfee;i="6700,10204,11222"; a="29660841" X-IronPort-AV: E=Sophos;i="6.11,199,1725346800"; d="scan'208";a="29660841" Received: from fmviesa006.fm.intel.com ([10.60.135.146]) by orvoesa112.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Oct 2024 11:30:31 -0700 X-CSE-ConnectionGUID: r28WnsQuR4eT47YGLF0Fuw== X-CSE-MsgGUID: d9pY0IawRF+iMbcD7A6TcQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.11,241,1725346800"; d="scan'208";a="81605272" Received: from ccbilbre-mobl3.amr.corp.intel.com (HELO [10.124.223.38]) ([10.124.223.38]) by fmviesa006-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Oct 2024 11:30:30 -0700 Message-ID: Date: Tue, 29 Oct 2024 11:30:30 -0700 Precedence: bulk X-Mailing-List: linux-pm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/3] x86/smp: Allow forcing the mwait hint for play dead loop To: Patryk Wlazlyn , x86@kernel.org Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, rafael.j.wysocki@intel.com, len.brown@intel.com, artem.bityutskiy@linux.intel.com, dave.hansen@linux.intel.com References: <20241029101507.7188-1-patryk.wlazlyn@linux.intel.com> <20241029101507.7188-3-patryk.wlazlyn@linux.intel.com> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/29/24 03:15, Patryk Wlazlyn wrote: > +void smp_set_mwait_play_dead_hint(unsigned int hint) > +{ > + WRITE_ONCE(play_dead_mwait_hint, hint); > +} This all feels a bit hacky and unstructured to me. Could we at least set up a few rules here? Like, say what the hints are, what values can they have? Where do they come from? Can this get called more than once? Does it _need_ to be set? What's the behavior when it is not set? Who is responsible for calling this? What good does the smp_ prefix do? I don't think _callers_ care whether this is getting optimized out or not. > - hint = get_deepest_mwait_hint(); > + hint = READ_ONCE(play_dead_mwait_hint); > + if (hint == PLAY_DEAD_MWAIT_HINT_UNSET) > + hint = get_deepest_mwait_hint(); This is also rather opaque. Why are there two hints? What makes one better than the other one?