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Thu, 6 Feb 2025 01:10:53 +0000 Received: from SA1PR10MB6365.namprd10.prod.outlook.com ([fe80::81bb:1fc4:37c7:a515]) by SA1PR10MB6365.namprd10.prod.outlook.com ([fe80::81bb:1fc4:37c7:a515%4]) with mapi id 15.20.8422.010; Thu, 6 Feb 2025 01:10:52 +0000 Message-ID: <85aa82ce-dde4-4ffa-8180-6616f72d6580@oracle.com> Date: Wed, 5 Feb 2025 17:10:47 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading .sframe headers To: Josh Poimboeuf Cc: Andrii Nakryiko , x86@kernel.org, Peter Zijlstra , Steven Rostedt , Ingo Molnar , Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org, Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , linux-perf-users@vger.kernel.org, Mark Brown , linux-toolchains@vger.kernel.org, Jordan Rome , Sam James , linux-trace-kernel@vger.kernel.org, Jens Remus , Mathieu Desnoyers , Florian Weimer , Andy Lutomirski , Masami Hiramatsu , Weinan Liu References: <20250124192159.ypvqwoqjvhasamev@jpoimboe> <47f1e244-992f-44fe-a0a5-6c271e9c719e@oracle.com> <20250129020249.owmklacvuvss7z7n@jpoimboe> <20250205005710.35wdzvobyhm6zyda@jpoimboe> Content-Language: en-US From: Indu Bhagat In-Reply-To: <20250205005710.35wdzvobyhm6zyda@jpoimboe> Content-Type: text/plain; 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For example: >>> >>> One insight is that the vast majority of (cfa, fp, ra) tuples aren't >>> unique. They could be deduped by storing the unique tuples in a >>> standalone 'fre_data' array which is referenced by another >>> address-specific array. >>> >>> struct fre_data { >>> s8|s16|s32 cfa, fp, ra; >>> u8 info; >>> }; >>> struct fre_data fre_data[num_fre_data]; >>> >> >> We had the same observation at the time of SFrame V1. And this method of >> compaction (deduped tuples) was brain-stormed a bit. Back then, the costs >> were thought to be: >> - more work at build time. >> - an additional data access once the FRE is found (as there is >> indirection). >> >> So it was really compaction at the costs above. We did steer towards >> simplicity and the SFrame FRE is what it stands today. >> >> The difference in the pros and cons now from then: >> - pros: helps mitigate unaligned accesses >> - cons: interferes slightly with the design goal of efficient addition and >> removal of stack trace information per function for JIT. Think "removal" as >> the set of actions necessary for addressing fragmentation in SFrame section >> data in JIT usecase. > > If fre_data[] is allowed to have duplicates then the deduping could be > optional. > >>> Note FDEs aren't even needed here as the unwinder doesn't need to know >>> when a function begins/ends. The only info needed by the unwinder is >>> just the fre_data struct. So a simple binary search of fres[] is all >>> that's really needed. >> >> Splitting out information (start_address) to an FDE (as done in V1/V2) has >> the benefit that a job like relocating information is proportional to >> O(NumFunctions). >> >> In the case above, IIUC, where the proposal puts start_address in the FRE, >> these costs will be (much) higher. > > I'm not sure I follow, is this referring to the link-time work of > sorting things? > I meant the work of tracking the start address of each function. This could be done at link-time as is done in most cases. But also depending on the case : e.g., kernel module loader will need to apply these relocations in the .rela.sframe section... If the granularity is finer than a function, more number of relocations will need to be applied. >> In addition, not being able to identify stack trace information per function >> will affect the JIT usecase. We need to able to mark stack trace >> information stale for functions in JIT environment. > > Maybe, though it's hard to really say how any of these changes would > affect JIT without knowing what those interfaces are going to look like. >