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Shutemov" , Michal Hocko , Boris Ostrovsky , Konrad Rzeszutek Wilk , Thomas Gleixner , Ingo Molnar , X86 ML , "H. Peter Anvin" , Arnd Bergmann , Andrew Morton , Ira Weiny , linux-arch References: <20201014195823.GC18196@zn.tnic> <22E29783-F1F5-43DA-B35F-D75FB247475D@amacapital.net> <20201014211214.GD18196@zn.tnic> From: Ankur Arora Message-ID: <3de58840-1f4c-566b-3a66-46d57475820c@oracle.com> Date: Wed, 14 Oct 2020 20:37:44 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20201014211214.GD18196@zn.tnic> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9774 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 spamscore=0 adultscore=0 bulkscore=0 mlxlogscore=999 malwarescore=0 mlxscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150025 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9774 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 clxscore=1015 impostorscore=0 phishscore=0 malwarescore=0 bulkscore=0 priorityscore=1501 mlxscore=0 suspectscore=0 spamscore=0 adultscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150025 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2020-10-14 2:12 p.m., Borislav Petkov wrote: > On Wed, Oct 14, 2020 at 02:07:30PM -0700, Andy Lutomirski wrote: >> I assume it=E2=80=99s for a little optimization of clearing more than = one >> page per SFENCE. >> >> In any event, based on the benchmark data upthread, we only want to do >> NT clears when they=E2=80=99re rather large, so this shouldn=E2=80=99t= be just an >> alternative. I assume this is because a page or two will fit in cache >> and, for most uses that allocate zeroed pages, we prefer cache-hot >> pages. When clearing 1G, on the other hand, cache-hot is impossible >> and we prefer the improved bandwidth and less cache trashing of NT >> clears. >=20 > Yeah, use case makes sense but people won't know what to use. At the > time I was experimenting with this crap, I remember Linus saying that > that selection should be made based on the size of the area cleared, so > users should not have to know the difference. I don't disagree but I think the selection of cached/uncached route shoul= d be made where we have enough context available to be able to choose to do this. This could be for example, done in mm_populate() or gup where if say the extent is larger than LLC-size, it takes the uncached path. >=20 > Which perhaps is the only sane use case I see for this. >=20 >> Perhaps SFENCE is so fast that this is a silly optimization, though, >> and we don=E2=80=99t lose anything measurable by SFENCEing once per pa= ge. >=20 > Yes, I'd like to see real use cases showing improvement from this, not > just microbenchmarks. Sure will add. Thanks Ankur >=20