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bulkscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2504070000 definitions=main-2504250073 X-Proofpoint-GUID: fDVeoUbfZmHDEwQedPK35O-Zxwj0L0T- X-Proofpoint-ORIG-GUID: fDVeoUbfZmHDEwQedPK35O-Zxwj0L0T- X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNDI1MDA3NCBTYWx0ZWRfXx96kf/6d2rFh ANCsC7ehsR02BrILOUBKFbN5ftRXL6aT0DsVV6i9FLPbfloY7+KE4+q0+F6BJ6XWcdkEbptgqZN jqfbkfOXwDniWw1Pzhe58zKcNLKBcqlyQCPlPnI+cTinquqyBEZQRFWdM6Te8YKeM5ebhmeZ3VT NrEF6dTB6SXbuHr713mLKeTMc28I0ETssh/MwgqCXRhTq8QEuhvQ2lyu2U9V5z3k4JVTAQ+OOCW Z4wG1q+P9FVyZ4AlfHfle6kZShrO5N9qnlv6JsPr2ReBmYX2VAyiYsHJudyrSXQJAJGiWQZdt7t QBR/4MUijnpnBlYq5EYV3wRxDG6/OZUPlzJ8l2n2NOnFxVxZAAPL9eL6ji/cu8EPeXhREKEe+hv e4WSGXtY On Thu, Apr 24, 2025 at 12:28:37PM +0100, Pedro Falcato wrote: > On Thu, Apr 24, 2025 at 05:07:48PM +0900, Harry Yoo wrote: > > Overview > > ======== > > > > The slab destructor feature existed in early days of slab allocator(s). > > It was removed by the commit c59def9f222d ("Slab allocators: Drop support > > for destructors") in 2007 due to lack of serious use cases at that time. > > > > Eighteen years later, Mateusz Guzik proposed [1] re-introducing a slab > > constructor/destructor pair to mitigate the global serialization point > > (pcpu_alloc_mutex) that occurs when each slab object allocates and frees > > percpu memory during its lifetime. > > > > Consider mm_struct: it allocates two percpu regions (mm_cid and rss_stat), > > so each allocate–free cycle requires two expensive acquire/release on > > that mutex. > > > > We can mitigate this contention by retaining the percpu regions after > > the object is freed and releasing them only when the backing slab pages > > are freed. > > > > How to do this with slab constructors and destructors: the constructor > > allocates percpu memory, and the destructor frees it when the slab pages > > are reclaimed; this slightly alters the constructor’s semantics, > > as it can now fail. > > > > I really really really really don't like this. We're opening a pandora's box > of locking issues for slab deadlocks and other subtle issues. IMO the best > solution there would be, what, failing dtors? which says a lot about the whole > situation... > > Case in point: <...snip...> > Then there are obviously other problems like: whatever you're calling must > not ever require the slab allocator (directly or indirectly) and must not > do direct reclaim (ever!), at the risk of a deadlock. The pcpu allocator > is a no-go (AIUI!) already because of such issues. Could you please elaborate more on this? -- Cheers, Harry / Hyeonggon